I have a similiar gap (~25GB) and my understanding of this goes along the lines
of:
Directories are typically represented as linked-lists of some description. They
often grow, but never shrink. They are effectively files that change whenever
you change an entry in that directly. They have a minimum size on disk
(depending on your file system).
My top-level directories in my cpool are all ~150kb, some 4096 top-level
directories - thats 600MB right there. All of us have something like this
somewhere.
The directory "maps" (if you like) that contain the structure of each backup is
stored under the "pc" directory in the root "data" directory. They have to be
on the same partition as your "cpool"/"pool" or hardlinks don't work.
Every backup of every PC consists of a number of directories. My backup summary
page tells me I have some ~512 full backups and around 2000 odd incrementals,
plus I have at least 50+ hosts in there, all with some logs, often 50MB+ of
logs per host.
Each one of the backup sets contains full directory maps (the file system is
the database). It works fast, but the minimum size on disk for many of those
directories is 4k (in my case).
Additionally, each of those directories contains an attrib file, which has the
ACLs and other bits.
Anyway, if each of those structures (per host/backup) adds up to ~10MB of
directories and attrib files, 2000 times 10mb is in the ball park of making up
for my "missing" ~25GB.
There are certain hosts (mailspools using maildir) that are going to aggravate
this issue (lots of large directories with lots of small files!).
I'm pretty sure that the performance penalty for using the tail-packing option
on a file system such as ReiserFS would lead to misery and pain (ie I don't
know about everyone else, but I have enough problems making my backups fit
inside a 24 hour period without further impacting disk IO).
I'm pretty sure I only need ~1000 directories (or less, if I take the
attributes into account) at 10kbyte each (which seems typical looking at my
directories) per backup (I have 2000+ individual backups) - 2000 x 1000 x 10kb
=~ 20GB
my 2c :) (actually, looks a bit more like 20c don't it).
//Chris
----- Original Message -----
From: "Saturn2888" <backuppc-forum AT backupcentral DOT com>
To: backuppc-users AT lists.sourceforge DOT net
Sent: Sunday, 19 September, 2010 9:27:00 PM
Subject: [BackupPC-users] Why is the BackupPC pool size 30GB less than the size
on the
Pool is 388.93GB comprising 2139798 files and 4369 directories (as of
2010-09-19 02:38),
Pool file system was recently at 77% (2010-09-19 08:23)
But df -h shows:
/dev/mapper/vg-backuppc 576G 420G 128G 77% /var/lib/backuppc
The volume group is only for BackupPC's pool, nothing else.
There's a pretty large difference between 390GB and 420GB.
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