Re: [linux-lvm] Data deduplication in LVM?

[Les Mikesell <lesmikesell@gmail.com>]

Les Mikesell wrote:
> Roy Sigurd Karlsbakk wrote:
>> On 11. juni. 2009, at 00.30, Stuart D. Gathman wrote:
>>
>>> One OSS backup product that does
>>> deduplication is BackupPC (written in Perl).  In the backup server, 
>>> every file
>>> gets hard linked to a name in a special directory that is its md5 
>>> checksum
>>> (plus some fiddly logic to handle metadata)
>>
>>
>> This sounds like file-level deduplication. Most storage systems sing 
>> dedup, uses block-level dedup. NetApp is one example; they dedup 
>> everything with 4k blocks, doing the actual deduplication at night.
> 
> Yes, it is a different concept.  However it does work very well when you 
> are storing your backups on a filesystem without block-level dedup.  And 
> that is probably the place where you have the most redundancy - or if 
> you don't already, you'll be able to store a much longer history.

Apologies for following up my own post, but this does remind me of a 
slightly related problem that someone here might have solved.  The 
backuppc archive ends up containing such a large number of directory 
entries and hardlinks that it is typically impractical to copy by any 
file-oriented means or even rsync.   A recurring topic on the backuppc 
mail list is how to make a copy for offsite storage.

Personally I use a RAID1 created with 3 mirror members and periodically 
swap one out and resync, but that's not very elegant.  Is there a better 
way or one that could be incrementally updated across a WAN?  Does LVM 
have a mechanism like zfs's incremental snapshot send/receive? (Not sure 
if that would work either but it sounds promising).  Is there any other 
way to do a block-oriented remote copy?  Would LVM mirroring work as 
well or better than md-device raid?  The partition can stay mounted 
while the raid rebuilds but realistically not much else can be happening 
because of the performance impact, and I unmount momentarily while 
removing the member to get a clean filesystem.

Are there tricks with drbd or perhaps raid over iscsi that would let a 
periodic sync work incrementally - well enough to use over a WAN?

-- 
   Les Mikesell
     lesmikesell@gmail.com