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[BackupPC-users] BackupPC and RAM (Was: Re: BackupPC on XFS getting lots of error -4 when calling...)
2011-05-24 15:52:58
Les Mikesell <lesmikesell AT gmail DOT com> wrote on 05/24/2011
11:09:09 AM:
> On 5/24/2011 9:36 AM, Timothy J Massey wrote:
> >
> > I ended up upgrading that system to 2GB of RAM merely so that
I could
> > finish the fsck.
> >
> > (Oh, and to a long-ago debate about would more RAM help BackupPC
to do
> > its job: nope. The backups with 2GB took almost exactly the same
amount
> > of time as the ones with 512MB. I wasn't swapping with 512MB,
and there
> > just isn't that much data that can be profitably cached while
doing a
> > backup, as long as the file list can fit in RAM: the few dentries
and
> > inodes in use at a time just don't take that much space...)
>
> That just seems wrong - although I'd consider 2Gb to be a fairly small
> amount of RAM and maybe not enough to help. It should be useful
to
> cache the whole pool directory tree so you don't have to seek there
to
> check for every hash match.
Two things: 1) If *quadrupuling* the total memory
does not improve the performance measurably, it is likely not to be improved
with even more, and 2) your idea of "small" and mine are
more than a little different. My backup servers are a *lot* closer
to Jeffrey's plug computer than whatever you're using. I guess there's
a third point: I'm not convinced that I'm *not* caching the entire
pool tree--especially with the 2GB.
I just reviewed my backup server logs. I upgraded
the RAM in 2011, and I've gone back and checked several different monthly
logs from 2010: all of the full backups were within 5% of each other
(and usually closer: this is an effectively static and unused server
with 250GB of data and about a half-million files).
So I just don't see that more RAM is going to help.
Period. Of course, YMMV (especially if you have a truly obscene
number of files: many millions).
By the way, to be clear: BackupPC does an outstanding
job even on small hardware: 1.2GHz VIA CPU, 512MB RAM and a single
SATA spindle is my standard BackupPC appliance (not server: appliance!
:) ). While I would certainly take it, I'm not actually *looking*
for more performance. My fulls take about 400 minutes for servers
with 250-500GB of data and 500,000 or so files, and my incrementals take
just minutes (15 - 75, depending on how late in the week/busy the users
are).
That's an effective backup rate of 19MB/s (with no
small thanks to rsync's help, too).
Timothy J. Massey
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