resurrect provisioning.py
authorBrian Warner <warner@lothar.com>
Tue, 26 Mar 2013 00:57:01 +0000 (17:57 -0700)
committerBrian Warner <warner@lothar.com>
Tue, 26 Mar 2013 00:57:01 +0000 (17:57 -0700)
It looks like commit 916d26e7103208fa207259d62ce453a5a8b9acd0, in
addition to making a one-line fix for #1681, also deleted
misc/operations_helpers/provisioning/provisioning.py entirely. This
brings it back.

misc/operations_helpers/provisioning/provisioning.py [new file with mode: 0644]

diff --git a/misc/operations_helpers/provisioning/provisioning.py b/misc/operations_helpers/provisioning/provisioning.py
new file mode 100644 (file)
index 0000000..37acd16
--- /dev/null
@@ -0,0 +1,776 @@
+
+from nevow import inevow, rend, loaders, tags as T
+import math
+import util
+
+# factorial and binomial copied from
+# http://mail.python.org/pipermail/python-list/2007-April/435718.html
+
+def div_ceil(n, d):
+    """
+    The smallest integer k such that k*d >= n.
+    """
+    return (n/d) + (n%d != 0)
+
+def factorial(n):
+    """factorial(n): return the factorial of the integer n.
+    factorial(0) = 1
+    factorial(n) with n<0 is -factorial(abs(n))
+    """
+    result = 1
+    for i in xrange(1, abs(n)+1):
+        result *= i
+    assert n >= 0
+    return result
+
+def binomial(n, k):
+    assert 0 <= k <= n
+    if k == 0 or k == n:
+        return 1
+    # calculate n!/k! as one product, avoiding factors that
+    # just get canceled
+    P = k+1
+    for i in xrange(k+2, n+1):
+        P *= i
+    # if you are paranoid:
+    # C, rem = divmod(P, factorial(n-k))
+    # assert rem == 0
+    # return C
+    return P//factorial(n-k)
+
+class ProvisioningTool(rend.Page):
+    addSlash = True
+    docFactory = loaders.xmlfile(util.sibling("provisioning.xhtml"))
+
+    def render_forms(self, ctx, data):
+        req = inevow.IRequest(ctx)
+
+        def getarg(name, astype=int):
+            if req.method != "POST":
+                return None
+            if name in req.fields:
+                return astype(req.fields[name].value)
+            return None
+        return self.do_forms(getarg)
+
+
+    def do_forms(self, getarg):
+        filled = getarg("filled", bool)
+
+        def get_and_set(name, options, default=None, astype=int):
+            current_value = getarg(name, astype)
+            i_select = T.select(name=name)
+            for (count, description) in options:
+                count = astype(count)
+                if ((current_value is not None and count == current_value) or
+                    (current_value is None and count == default)):
+                    o = T.option(value=str(count), selected="true")[description]
+                else:
+                    o = T.option(value=str(count))[description]
+                i_select = i_select[o]
+            if current_value is None:
+                current_value = default
+            return current_value, i_select
+
+        sections = {}
+        def add_input(section, text, entry):
+            if section not in sections:
+                sections[section] = []
+            sections[section].extend([T.div[text, ": ", entry], "\n"])
+
+        def add_output(section, entry):
+            if section not in sections:
+                sections[section] = []
+            sections[section].extend([entry, "\n"])
+
+        def build_section(section):
+            return T.fieldset[T.legend[section], sections[section]]
+
+        def number(value, suffix=""):
+            scaling = 1
+            if value < 1:
+                fmt = "%1.2g%s"
+            elif value < 100:
+                fmt = "%.1f%s"
+            elif value < 1000:
+                fmt = "%d%s"
+            elif value < 1e6:
+                fmt = "%.2fk%s"; scaling = 1e3
+            elif value < 1e9:
+                fmt = "%.2fM%s"; scaling = 1e6
+            elif value < 1e12:
+                fmt = "%.2fG%s"; scaling = 1e9
+            elif value < 1e15:
+                fmt = "%.2fT%s"; scaling = 1e12
+            elif value < 1e18:
+                fmt = "%.2fP%s"; scaling = 1e15
+            else:
+                fmt = "huge! %g%s"
+            return fmt % (value / scaling, suffix)
+
+        user_counts = [(5, "5 users"),
+                       (50, "50 users"),
+                       (200, "200 users"),
+                       (1000, "1k users"),
+                       (10000, "10k users"),
+                       (50000, "50k users"),
+                       (100000, "100k users"),
+                       (500000, "500k users"),
+                       (1000000, "1M users"),
+                       ]
+        num_users, i_num_users = get_and_set("num_users", user_counts, 50000)
+        add_input("Users",
+                  "How many users are on this network?", i_num_users)
+
+        files_per_user_counts = [(100, "100 files"),
+                                 (1000, "1k files"),
+                                 (10000, "10k files"),
+                                 (100000, "100k files"),
+                                 (1e6, "1M files"),
+                                 ]
+        files_per_user, i_files_per_user = get_and_set("files_per_user",
+                                                       files_per_user_counts,
+                                                       1000)
+        add_input("Users",
+                  "How many files for each user? (avg)",
+                  i_files_per_user)
+
+        space_per_user_sizes = [(1e6, "1MB"),
+                                (10e6, "10MB"),
+                                (100e6, "100MB"),
+                                (200e6, "200MB"),
+                                (1e9, "1GB"),
+                                (2e9, "2GB"),
+                                (5e9, "5GB"),
+                                (10e9, "10GB"),
+                                (100e9, "100GB"),
+                                (1e12, "1TB"),
+                                (2e12, "2TB"),
+                                (5e12, "5TB"),
+                                ]
+        # Estimate ~5gb per user as a more realistic case
+        space_per_user, i_space_per_user = get_and_set("space_per_user",
+                                                       space_per_user_sizes,
+                                                       5e9)
+        add_input("Users",
+                  "How much data for each user? (avg)",
+                  i_space_per_user)
+
+        sharing_ratios = [(1.0, "1.0x"),
+                          (1.1, "1.1x"),
+                          (2.0, "2.0x"),
+                          ]
+        sharing_ratio, i_sharing_ratio = get_and_set("sharing_ratio",
+                                                     sharing_ratios, 1.0,
+                                                     float)
+        add_input("Users",
+                  "What is the sharing ratio? (1.0x is no-sharing and"
+                  " no convergence)", i_sharing_ratio)
+
+        # Encoding parameters
+        encoding_choices = [("3-of-10-5", "3.3x (3-of-10, repair below 5)"),
+                            ("3-of-10-8", "3.3x (3-of-10, repair below 8)"),
+                            ("5-of-10-7", "2x (5-of-10, repair below 7)"),
+                            ("8-of-10-9", "1.25x (8-of-10, repair below 9)"),
+                            ("27-of-30-28", "1.1x (27-of-30, repair below 28"),
+                            ("25-of-100-50", "4x (25-of-100, repair below 50)"),
+                            ]
+        encoding_parameters, i_encoding_parameters = \
+                             get_and_set("encoding_parameters",
+                                         encoding_choices, "3-of-10-5", str)
+        encoding_pieces = encoding_parameters.split("-")
+        k = int(encoding_pieces[0])
+        assert encoding_pieces[1] == "of"
+        n = int(encoding_pieces[2])
+        # we repair the file when the number of available shares drops below
+        # this value
+        repair_threshold = int(encoding_pieces[3])
+
+        add_input("Servers",
+                  "What are the default encoding parameters?",
+                  i_encoding_parameters)
+
+        # Server info
+        num_server_choices = [ (5, "5 servers"),
+                               (10, "10 servers"),
+                               (15, "15 servers"),
+                               (30, "30 servers"),
+                               (50, "50 servers"),
+                               (100, "100 servers"),
+                               (200, "200 servers"),
+                               (300, "300 servers"),
+                               (500, "500 servers"),
+                               (1000, "1k servers"),
+                               (2000, "2k servers"),
+                               (5000, "5k servers"),
+                               (10e3, "10k servers"),
+                               (100e3, "100k servers"),
+                               (1e6, "1M servers"),
+                               ]
+        num_servers, i_num_servers = \
+                     get_and_set("num_servers", num_server_choices, 30, int)
+        add_input("Servers",
+                  "How many servers are there?", i_num_servers)
+
+        # availability is measured in dBA = -dBF, where 0dBF is 100% failure,
+        # 10dBF is 10% failure, 20dBF is 1% failure, etc
+        server_dBA_choices = [ (10, "90% [10dBA] (2.4hr/day)"),
+                               (13, "95% [13dBA] (1.2hr/day)"),
+                               (20, "99% [20dBA] (14min/day or 3.5days/year)"),
+                               (23, "99.5% [23dBA] (7min/day or 1.75days/year)"),
+                               (30, "99.9% [30dBA] (87sec/day or 9hours/year)"),
+                               (40, "99.99% [40dBA] (60sec/week or 53min/year)"),
+                               (50, "99.999% [50dBA] (5min per year)"),
+                               ]
+        server_dBA, i_server_availability = \
+                    get_and_set("server_availability",
+                                server_dBA_choices,
+                                20, int)
+        add_input("Servers",
+                  "What is the server availability?", i_server_availability)
+
+        drive_MTBF_choices = [ (40, "40,000 Hours"),
+                               ]
+        drive_MTBF, i_drive_MTBF = \
+                    get_and_set("drive_MTBF", drive_MTBF_choices, 40, int)
+        add_input("Drives",
+                  "What is the hard drive MTBF?", i_drive_MTBF)
+        # http://www.tgdaily.com/content/view/30990/113/
+        # http://labs.google.com/papers/disk_failures.pdf
+        # google sees:
+        #  1.7% of the drives they replaced were 0-1 years old
+        #  8% of the drives they repalced were 1-2 years old
+        #  8.6% were 2-3 years old
+        #  6% were 3-4 years old, about 8% were 4-5 years old
+
+        drive_size_choices = [ (100, "100 GB"),
+                               (250, "250 GB"),
+                               (500, "500 GB"),
+                               (750, "750 GB"),
+                               (1000, "1000 GB"),
+                               (2000, "2000 GB"),
+                               (3000, "3000 GB"),
+                               ]
+        drive_size, i_drive_size = \
+                    get_and_set("drive_size", drive_size_choices, 3000, int)
+        drive_size = drive_size * 1e9
+        add_input("Drives",
+                  "What is the capacity of each hard drive?", i_drive_size)
+        drive_failure_model_choices = [ ("E", "Exponential"),
+                                        ("U", "Uniform"),
+                                        ]
+        drive_failure_model, i_drive_failure_model = \
+                             get_and_set("drive_failure_model",
+                                         drive_failure_model_choices,
+                                         "E", str)
+        add_input("Drives",
+                  "How should we model drive failures?", i_drive_failure_model)
+
+        # drive_failure_rate is in failures per second
+        if drive_failure_model == "E":
+            drive_failure_rate = 1.0 / (drive_MTBF * 1000 * 3600)
+        else:
+            drive_failure_rate = 0.5 / (drive_MTBF * 1000 * 3600)
+
+        # deletion/gc/ownership mode
+        ownership_choices = [ ("A", "no deletion, no gc, no owners"),
+                              ("B", "deletion, no gc, no owners"),
+                              ("C", "deletion, share timers, no owners"),
+                              ("D", "deletion, no gc, yes owners"),
+                              ("E", "deletion, owner timers"),
+                              ]
+        ownership_mode, i_ownership_mode = \
+                        get_and_set("ownership_mode", ownership_choices,
+                                    "A", str)
+        add_input("Servers",
+                  "What is the ownership mode?", i_ownership_mode)
+
+        # client access behavior
+        access_rates = [ (1, "one file per day"),
+                         (10, "10 files per day"),
+                         (100, "100 files per day"),
+                         (1000, "1k files per day"),
+                         (10e3, "10k files per day"),
+                         (100e3, "100k files per day"),
+                         ]
+        download_files_per_day, i_download_rate = \
+                                get_and_set("download_rate", access_rates,
+                                            100, int)
+        add_input("Users",
+                  "How many files are downloaded per day?", i_download_rate)
+        download_rate = 1.0 * download_files_per_day / (24*60*60)
+
+        upload_files_per_day, i_upload_rate = \
+                              get_and_set("upload_rate", access_rates,
+                                          10, int)
+        add_input("Users",
+                  "How many files are uploaded per day?", i_upload_rate)
+        upload_rate = 1.0 * upload_files_per_day / (24*60*60)
+
+        delete_files_per_day, i_delete_rate = \
+                              get_and_set("delete_rate", access_rates,
+                                          10, int)
+        add_input("Users",
+                  "How many files are deleted per day?", i_delete_rate)
+        delete_rate = 1.0 * delete_files_per_day / (24*60*60)
+
+
+        # the value is in days
+        lease_timers = [ (1, "one refresh per day"),
+                         (7, "one refresh per week"),
+                         ]
+        lease_timer, i_lease = \
+                     get_and_set("lease_timer", lease_timers,
+                                 7, int)
+        add_input("Users",
+                  "How frequently do clients refresh files or accounts? "
+                  "(if necessary)",
+                  i_lease)
+        seconds_per_lease = 24*60*60*lease_timer
+
+        check_timer_choices = [ (1, "every week"),
+                                (4, "every month"),
+                                (8, "every two months"),
+                                (16, "every four months"),
+                                ]
+        check_timer, i_check_timer = \
+                     get_and_set("check_timer", check_timer_choices, 4, int)
+        add_input("Users",
+                  "How frequently should we check on each file?",
+                  i_check_timer)
+        file_check_interval = check_timer * 7 * 24 * 3600
+
+
+        if filled:
+            add_output("Users", T.div["Total users: %s" % number(num_users)])
+            add_output("Users",
+                       T.div["Files per user: %s" % number(files_per_user)])
+            file_size = 1.0 * space_per_user / files_per_user
+            add_output("Users",
+                       T.div["Average file size: ", number(file_size)])
+            total_files = num_users * files_per_user / sharing_ratio
+
+            add_output("Grid",
+                       T.div["Total number of files in grid: ",
+                             number(total_files)])
+            total_space = num_users * space_per_user / sharing_ratio
+            add_output("Grid",
+                       T.div["Total volume of plaintext in grid: ",
+                             number(total_space, "B")])
+
+            total_shares = n * total_files
+            add_output("Grid",
+                       T.div["Total shares in grid: ", number(total_shares)])
+            expansion = float(n) / float(k)
+
+            total_usage = expansion * total_space
+            add_output("Grid",
+                       T.div["Share data in grid: ", number(total_usage, "B")])
+
+            if n > num_servers:
+                # silly configuration, causes Tahoe2 to wrap and put multiple
+                # shares on some servers.
+                add_output("Servers",
+                           T.div["non-ideal: more shares than servers"
+                                 " (n=%d, servers=%d)" % (n, num_servers)])
+                # every file has at least one share on every server
+                buckets_per_server = total_files
+                shares_per_server = total_files * ((1.0 * n) / num_servers)
+            else:
+                # if nobody is full, then no lease requests will be turned
+                # down for lack of space, and no two shares for the same file
+                # will share a server. Therefore the chance that any given
+                # file has a share on any given server is n/num_servers.
+                buckets_per_server = total_files * ((1.0 * n) / num_servers)
+                # since each such represented file only puts one share on a
+                # server, the total number of shares per server is the same.
+                shares_per_server = buckets_per_server
+            add_output("Servers",
+                       T.div["Buckets per server: ",
+                             number(buckets_per_server)])
+            add_output("Servers",
+                       T.div["Shares per server: ",
+                             number(shares_per_server)])
+
+            # how much space is used on the storage servers for the shares?
+            #  the share data itself
+            share_data_per_server = total_usage / num_servers
+            add_output("Servers",
+                       T.div["Share data per server: ",
+                             number(share_data_per_server, "B")])
+            # this is determined empirically. H=hashsize=32, for a one-segment
+            # file and 3-of-10 encoding
+            share_validation_per_server = 266 * shares_per_server
+            # this could be 423*buckets_per_server, if we moved the URI
+            # extension into a separate file, but that would actually consume
+            # *more* space (minimum filesize is 4KiB), unless we moved all
+            # shares for a given bucket into a single file.
+            share_uri_extension_per_server = 423 * shares_per_server
+
+            # ownership mode adds per-bucket data
+            H = 32 # depends upon the desired security of delete/refresh caps
+            # bucket_lease_size is the amount of data needed to keep track of
+            # the delete/refresh caps for each bucket.
+            bucket_lease_size = 0
+            client_bucket_refresh_rate = 0
+            owner_table_size = 0
+            if ownership_mode in ("B", "C", "D", "E"):
+                bucket_lease_size = sharing_ratio * 1.0 * H
+            if ownership_mode in ("B", "C"):
+                # refreshes per second per client
+                client_bucket_refresh_rate = (1.0 * n * files_per_user /
+                                              seconds_per_lease)
+                add_output("Users",
+                           T.div["Client share refresh rate (outbound): ",
+                                 number(client_bucket_refresh_rate, "Hz")])
+                server_bucket_refresh_rate = (client_bucket_refresh_rate *
+                                              num_users / num_servers)
+                add_output("Servers",
+                           T.div["Server share refresh rate (inbound): ",
+                                 number(server_bucket_refresh_rate, "Hz")])
+            if ownership_mode in ("D", "E"):
+                # each server must maintain a bidirectional mapping from
+                # buckets to owners. One way to implement this would be to
+                # put a list of four-byte owner numbers into each bucket, and
+                # a list of four-byte share numbers into each owner (although
+                # of course we'd really just throw it into a database and let
+                # the experts take care of the details).
+                owner_table_size = 2*(buckets_per_server * sharing_ratio * 4)
+
+            if ownership_mode in ("E",):
+                # in this mode, clients must refresh one timer per server
+                client_account_refresh_rate = (1.0 * num_servers /
+                                               seconds_per_lease)
+                add_output("Users",
+                           T.div["Client account refresh rate (outbound): ",
+                                 number(client_account_refresh_rate, "Hz")])
+                server_account_refresh_rate = (client_account_refresh_rate *
+                                              num_users / num_servers)
+                add_output("Servers",
+                           T.div["Server account refresh rate (inbound): ",
+                                 number(server_account_refresh_rate, "Hz")])
+
+            # TODO: buckets vs shares here is a bit wonky, but in
+            # non-wrapping grids it shouldn't matter
+            share_lease_per_server = bucket_lease_size * buckets_per_server
+            share_ownertable_per_server = owner_table_size
+
+            share_space_per_server = (share_data_per_server +
+                                      share_validation_per_server +
+                                      share_uri_extension_per_server +
+                                      share_lease_per_server +
+                                      share_ownertable_per_server)
+            add_output("Servers",
+                       T.div["Share space per server: ",
+                             number(share_space_per_server, "B"),
+                             " (data ",
+                             number(share_data_per_server, "B"),
+                             ", validation ",
+                             number(share_validation_per_server, "B"),
+                             ", UEB ",
+                             number(share_uri_extension_per_server, "B"),
+                             ", lease ",
+                             number(share_lease_per_server, "B"),
+                             ", ownertable ",
+                             number(share_ownertable_per_server, "B"),
+                             ")",
+                             ])
+
+
+            # rates
+            client_download_share_rate = download_rate * k
+            client_download_byte_rate = download_rate * file_size
+            add_output("Users",
+                       T.div["download rate: shares = ",
+                             number(client_download_share_rate, "Hz"),
+                             " , bytes = ",
+                             number(client_download_byte_rate, "Bps"),
+                             ])
+            total_file_check_rate = 1.0 * total_files / file_check_interval
+            client_check_share_rate = total_file_check_rate / num_users
+            add_output("Users",
+                       T.div["file check rate: shares = ",
+                             number(client_check_share_rate, "Hz"),
+                             " (interval = %s)" %
+                             number(1 / client_check_share_rate, "s"),
+                             ])
+
+            client_upload_share_rate = upload_rate * n
+            # TODO: doesn't include overhead
+            client_upload_byte_rate = upload_rate * file_size * expansion
+            add_output("Users",
+                       T.div["upload rate: shares = ",
+                             number(client_upload_share_rate, "Hz"),
+                             " , bytes = ",
+                             number(client_upload_byte_rate, "Bps"),
+                             ])
+            client_delete_share_rate = delete_rate * n
+
+            server_inbound_share_rate = (client_upload_share_rate *
+                                         num_users / num_servers)
+            server_inbound_byte_rate = (client_upload_byte_rate *
+                                        num_users / num_servers)
+            add_output("Servers",
+                       T.div["upload rate (inbound): shares = ",
+                             number(server_inbound_share_rate, "Hz"),
+                             " , bytes = ",
+                              number(server_inbound_byte_rate, "Bps"),
+                             ])
+            add_output("Servers",
+                       T.div["share check rate (inbound): ",
+                             number(total_file_check_rate * n / num_servers,
+                                    "Hz"),
+                             ])
+
+            server_share_modify_rate = ((client_upload_share_rate +
+                                         client_delete_share_rate) *
+                                         num_users / num_servers)
+            add_output("Servers",
+                       T.div["share modify rate: shares = ",
+                             number(server_share_modify_rate, "Hz"),
+                             ])
+
+            server_outbound_share_rate = (client_download_share_rate *
+                                          num_users / num_servers)
+            server_outbound_byte_rate = (client_download_byte_rate *
+                                         num_users / num_servers)
+            add_output("Servers",
+                       T.div["download rate (outbound): shares = ",
+                             number(server_outbound_share_rate, "Hz"),
+                             " , bytes = ",
+                              number(server_outbound_byte_rate, "Bps"),
+                             ])
+
+
+            total_share_space = num_servers * share_space_per_server
+            add_output("Grid",
+                       T.div["Share space consumed: ",
+                             number(total_share_space, "B")])
+            add_output("Grid",
+                       T.div[" %% validation: %.2f%%" %
+                             (100.0 * share_validation_per_server /
+                              share_space_per_server)])
+            add_output("Grid",
+                       T.div[" %% uri-extension: %.2f%%" %
+                             (100.0 * share_uri_extension_per_server /
+                              share_space_per_server)])
+            add_output("Grid",
+                       T.div[" %% lease data: %.2f%%" %
+                             (100.0 * share_lease_per_server /
+                              share_space_per_server)])
+            add_output("Grid",
+                       T.div[" %% owner data: %.2f%%" %
+                             (100.0 * share_ownertable_per_server /
+                              share_space_per_server)])
+            add_output("Grid",
+                       T.div[" %% share data: %.2f%%" %
+                             (100.0 * share_data_per_server /
+                              share_space_per_server)])
+            add_output("Grid",
+                       T.div["file check rate: ",
+                             number(total_file_check_rate,
+                                    "Hz")])
+
+            total_drives = max(div_ceil(int(total_share_space),
+                                        int(drive_size)),
+                               num_servers)
+            add_output("Drives",
+                       T.div["Total drives: ", number(total_drives), " drives"])
+            drives_per_server = div_ceil(total_drives, num_servers)
+            add_output("Servers",
+                       T.div["Drives per server: ", drives_per_server])
+
+            # costs
+            if drive_size == 3000 * 1e9:
+                add_output("Servers", T.div["3000GB drive: $250 each"])
+                drive_cost = 250
+            else:
+                add_output("Servers",
+                           T.div[T.b["unknown cost per drive, assuming $100"]])
+                drive_cost = 100
+
+            if drives_per_server <= 4:
+                add_output("Servers", T.div["1U box with <= 4 drives: $1500"])
+                server_cost = 1500 # typical 1U box
+            elif drives_per_server <= 12:
+                add_output("Servers", T.div["2U box with <= 12 drives: $2500"])
+                server_cost = 2500 # 2U box
+            else:
+                add_output("Servers",
+                           T.div[T.b["Note: too many drives per server, "
+                                     "assuming $3000"]])
+                server_cost = 3000
+
+            server_capital_cost = (server_cost + drives_per_server * drive_cost)
+            total_server_cost = float(num_servers * server_capital_cost)
+            add_output("Servers", T.div["Capital cost per server: $",
+                                        server_capital_cost])
+            add_output("Grid", T.div["Capital cost for all servers: $",
+                                     number(total_server_cost)])
+            # $70/Mbps/mo
+            # $44/server/mo power+space
+            server_bandwidth = max(server_inbound_byte_rate,
+                                   server_outbound_byte_rate)
+            server_bandwidth_mbps = div_ceil(int(server_bandwidth*8), int(1e6))
+            server_monthly_cost = 70*server_bandwidth_mbps + 44
+            add_output("Servers", T.div["Monthly cost per server: $",
+                                        server_monthly_cost])
+            add_output("Users", T.div["Capital cost per user: $",
+                                      number(total_server_cost / num_users)])
+
+            # reliability
+            any_drive_failure_rate = total_drives * drive_failure_rate
+            any_drive_MTBF = 1 // any_drive_failure_rate  # in seconds
+            any_drive_MTBF_days = any_drive_MTBF / 86400
+            add_output("Drives",
+                       T.div["MTBF (any drive): ",
+                             number(any_drive_MTBF_days), " days"])
+            drive_replacement_monthly_cost = (float(drive_cost)
+                                              * any_drive_failure_rate
+                                              *30*86400)
+            add_output("Grid",
+                       T.div["Monthly cost of replacing drives: $",
+                             number(drive_replacement_monthly_cost)])
+
+            total_server_monthly_cost = float(num_servers * server_monthly_cost
+                                              + drive_replacement_monthly_cost)
+
+            add_output("Grid", T.div["Monthly cost for all servers: $",
+                                     number(total_server_monthly_cost)])
+            add_output("Users",
+                       T.div["Monthly cost per user: $",
+                             number(total_server_monthly_cost / num_users)])
+
+            # availability
+            file_dBA = self.file_availability(k, n, server_dBA)
+            user_files_dBA = self.many_files_availability(file_dBA,
+                                                          files_per_user)
+            all_files_dBA = self.many_files_availability(file_dBA, total_files)
+            add_output("Users",
+                       T.div["availability of: ",
+                             "arbitrary file = %d dBA, " % file_dBA,
+                             "all files of user1 = %d dBA, " % user_files_dBA,
+                             "all files in grid = %d dBA" % all_files_dBA,
+                             ],
+                       )
+
+            time_until_files_lost = (n-k+1) / any_drive_failure_rate
+            add_output("Grid",
+                       T.div["avg time until files are lost: ",
+                             number(time_until_files_lost, "s"), ", ",
+                             number(time_until_files_lost/86400, " days"),
+                             ])
+
+            share_data_loss_rate = any_drive_failure_rate * drive_size
+            add_output("Grid",
+                       T.div["share data loss rate: ",
+                             number(share_data_loss_rate,"Bps")])
+
+            # the worst-case survival numbers occur when we do a file check
+            # and the file is just above the threshold for repair (so we
+            # decide to not repair it). The question is then: what is the
+            # chance that the file will decay so badly before the next check
+            # that we can't recover it? The resulting probability is per
+            # check interval.
+            # Note that the chances of us getting into this situation are low.
+            P_disk_failure_during_interval = (drive_failure_rate *
+                                              file_check_interval)
+            disk_failure_dBF = 10*math.log10(P_disk_failure_during_interval)
+            disk_failure_dBA = -disk_failure_dBF
+            file_survives_dBA = self.file_availability(k, repair_threshold,
+                                                       disk_failure_dBA)
+            user_files_survives_dBA = self.many_files_availability( \
+                file_survives_dBA, files_per_user)
+            all_files_survives_dBA = self.many_files_availability( \
+                file_survives_dBA, total_files)
+            add_output("Users",
+                       T.div["survival of: ",
+                             "arbitrary file = %d dBA, " % file_survives_dBA,
+                             "all files of user1 = %d dBA, " %
+                             user_files_survives_dBA,
+                             "all files in grid = %d dBA" %
+                             all_files_survives_dBA,
+                             " (per worst-case check interval)",
+                             ])
+
+
+
+        all_sections = []
+        all_sections.append(build_section("Users"))
+        all_sections.append(build_section("Servers"))
+        all_sections.append(build_section("Drives"))
+        if "Grid" in sections:
+            all_sections.append(build_section("Grid"))
+
+        f = T.form(action=".", method="post", enctype="multipart/form-data")
+
+        if filled:
+            action = "Recompute"
+        else:
+            action = "Compute"
+
+        f = f[T.input(type="hidden", name="filled", value="true"),
+              T.input(type="submit", value=action),
+              all_sections,
+              ]
+
+        try:
+            from allmydata import reliability
+            # we import this just to test to see if the page is available
+            _hush_pyflakes = reliability
+            del _hush_pyflakes
+            f = [T.div[T.a(href="../reliability")["Reliability Math"]], f]
+        except ImportError:
+            pass
+
+        return f
+
+    def file_availability(self, k, n, server_dBA):
+        """
+        The full formula for the availability of a specific file is::
+
+         1 - sum([choose(N,i) * p**i * (1-p)**(N-i)] for i in range(k)])
+
+        Where choose(N,i) = N! / ( i! * (N-i)! ) . Note that each term of
+        this summation is the probability that there are exactly 'i' servers
+        available, and what we're doing is adding up the cases where i is too
+        low.
+
+        This is a nuisance to calculate at all accurately, especially once N
+        gets large, and when p is close to unity. So we make an engineering
+        approximation: if (1-p) is very small, then each [i] term is much
+        larger than the [i-1] term, and the sum is dominated by the i=k-1
+        term. This only works for (1-p) < 10%, and when the choose() function
+        doesn't rise fast enough to compensate. For high-expansion encodings
+        (3-of-10, 25-of-100), the choose() function is rising at the same
+        time as the (1-p)**(N-i) term, so that's not an issue. For
+        low-expansion encodings (7-of-10, 75-of-100) the two values are
+        moving in opposite directions, so more care must be taken.
+
+        Note that the p**i term has only a minor effect as long as (1-p)*N is
+        small, and even then the effect is attenuated by the 1-p term.
+        """
+
+        assert server_dBA > 9  # >=90% availability to use the approximation
+        factor = binomial(n, k-1)
+        factor_dBA = 10 * math.log10(factor)
+        exponent = n - k + 1
+        file_dBA = server_dBA * exponent - factor_dBA
+        return file_dBA
+
+    def many_files_availability(self, file_dBA, num_files):
+        """The probability that 'num_files' independent bernoulli trials will
+        succeed (i.e. we can recover all files in the grid at any given
+        moment) is p**num_files . Since p is close to unity, we express in p
+        in dBA instead, so we can get useful precision on q (=1-p), and then
+        the formula becomes::
+
+         P_some_files_unavailable = 1 - (1 - q)**num_files
+
+        That (1-q)**n expands with the usual binomial sequence, 1 - nq +
+        Xq**2 ... + Xq**n . We use the same approximation as before, since we
+        know q is close to zero, and we get to ignore all the terms past -nq.
+        """
+
+        many_files_dBA = file_dBA - 10 * math.log10(num_files)
+        return many_files_dBA