docs/filesystem-notes.txt: add notes about enabling the 'directory index' feature...

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+
+Tahoe storage servers use a large number of subdirectories to store their
+shares on local disk. This format is simple and robust, but depends upon the
+local filesystem to provide fast access to those directories.
+
+= ext3 =
+
+For moderate- or large-sized storage servers, you'll want to make sure the
+"directory index" feature is enabled on your ext3 directories, otherwise
+share lookup may be very slow. Recent versions of ext3 enable this
+automatically, but older filesystems may not have it enabled.
+
+$ sudo tune2fs -l /dev/sda1 |grep feature
+Filesystem features:      has_journal ext_attr resize_inode dir_index filetype needs_recovery sparse_super large_file
+
+If "dir_index" is present in the "features:" line, then you're all set. If
+not, you'll need to use tune2fs and e2fsck to enable and build the index. See
+this page for some hints: http://wiki.dovecot.org/MailboxFormat/Maildir .