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Re: [BackupPC-users] Centralized storage with multiple hard drives
2014-03-20 15:00:09
Les Mikesell <lesmikesell AT gmail DOT com> wrote on 03/20/2014
01:59:47 PM:
> On Thu, Mar 20, 2014 at 12:21 PM, Timothy J Massey
> <tmassey AT obscorp DOT com> wrote:
> >
> > >
> > > You've used BackupPC before, Les, right? ;-)
> > > BackupPC prefers pool reads over writes when possible, and
it typically
> > > accesses large amounts of data almost randomly. Caching
metadatawill help,
> > > caching data likely won't.
> >
> >
> > I've given up explaining this to Mr. Mikesell. I've posted
info
> to the list several times showing the performance on BackupPC
> servers that had 512MB (Megabytes) of RAM. Zero swapping, and
> something like 300MB used for caching. I have then upgraded
them to
> 4GB of RAM (8 times as much RAM!) and saw a whopping 15 or so
> minutes savings on a backup that took 10 to 12 hours. (In fact,
the
> only real reason I use 4GB minimum across the board now is that I
> ran into a problem where a fsck wouldn't complete without more RAM.)
>
> 4GB is still a tiny amount of RAM these days. Try 20+. You
basically
> want to cache all of your pool directory/inode data plus the pc backup
> trees being traversed by rsync. And without threshing it out
with the
> rsync copy of the remote trees that are being processed.
After this, I *really* give up.
I *have* servers with 32GB of RAM. I've gone
from 4GB to 16GB and seen zero performance increase.
Do you *REALLY* think this is some sort of binary
on/off enhancement? Have 4GB? No benefit. 5GB? WOW
Look at it go!!! If you see zero improvement with 8 TIMES AS MUCH
RAM (when you're not swapping to begin with!), you expect me to believe
that double or triple that and all of a sudden a big difference? Come
on now...
> > And I haven't seen that RAID-5 has been that much of an issue.
As
> you mention, BackupPC is *READ* heavy so the RMW penalty doesn't
> hurt much. My experience with both RAID-5 *AND* RAID-6 (even
> software RAID-6!) is just fine.
>
> Sure, they work. But they force all of the disk heads to seek
every
> time unless you have a large number of disks, and every short write
> (and most of them are with all of the directory/inode operations
> happening), is going to cost an extra disk revolution time for the
> read/modify/write step. And that 'read heavy' assessment is
somewhat
> optimistic if you have any large files that are modified in place.
> There you get the worst case of alternating small reads/writes in
> different places as the changes delivered by rsync are merged into
a
> copy of the old file.
I'm not saying that there isn't a penalty for RAID-5
or -6. I'm saying that the penalty is usually NOT RELEVANT FOR THIS
APPLICATION! Not in theory, but in ACTUAL PRACTICE. You then
go on to make my point:
> > Of course, I have a minimum of 4 drives
in a RAID array (6 minimum
> for RAID-6), so I'm usually bottlenecked somewhere else anyway, such
> as a single 1Gb link. It's not hard to mange writing 70MB/s
of data!
>
> Seriously? You have rsync pegging a 1Gb link for long periods
of time
> anytime but the first run? Do you have something creating a
lot of
> huge new files? My times are mostly constrained by the read
time on
> the targets with a relatively small amount of data transfer.
You have stated that you're limited by the TARGET.
So given that, who cares if the disks are 60% busy instead of 30%
busy while it waits on those targets? And why WOULDN'T I use something
like RAID-5 or -6 and give myself a LOT more usable space?
And my point was not that I *am* limited by my 1GbE
link (I'm not usually with BackupPC but I *am* with other uses of my backup
servers in general), but that at *BEST* I can move 70MB/s *because* I'm
using 1GbE. Seeing as my 6-drive RAID-6 array can copy files from
one spot on the array to another (so not just simply streaming reads or
writes) at 400MB/s routinely (and >1,000MB on my 12-drive systems),
who *CARES* how much faster it would be if it were, say, RAID-10? This
server even has 4 x 1GbE, but you *never* get that with bonded Ethernet.
You're lucky to get 150MB/s. So my drives are NEVER THE ISSUE!
So, yes, I'll take the extra space, which is *ALWAYS*
my limiting factor! :)
Tim Massey
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