Sorin Srbu wrote:
>> -----Original Message-----
>> From: Les Mikesell [mailto:lesmikesell AT gmail DOT com]
>> Sent: Tuesday, May 11, 2010 5:07 PM
>> To: backuppc-users AT lists.sourceforge DOT net
>> Subject: Re: [BackupPC-users] [Solved] Slow web-GUI access
>>
>> Read access is normally OK on a raid5.  The issues are that with a small
>> number of drives you essentially force all the heads to seek together to
>> handle writing the parity and on partial block writes you have to read
>> first, then rewrite to update the parity.  So you slow down to the seek
>> time of the slowest drive.  If you noticed speed issues even when
>> backups were not writing to the array you probably had some other
>> problem.  Did you check (cat /proc/mdstat) to make sure all the drives
>> were working?  Perhaps it was running degraded and your eventual failure
>> was when a 2nd drive dropped out.
> 
> So was my understanding too; that raid5 read performance was ok.
> 
> What is a small number of drives? I have three identical 500GB Seagates of 
> the 
> SATA2 variety, all bought new late last year, so I'm assuming fairly modern 
> as 
> well.

Something less than a dozen is a small number.  With 3, every write is going to 
require every disk head to seek and you'll wait until the slowest one completes 
before the next operation.

> My checks in /proc/mdstat showed nothing unusual. They've been working fine 
> all the time, all three of them; ie none is failed.
> 
> The backup of the linux machines here, is for the most part hundreds of 
> thousands of really small files, like in the 10-100kB-range. Could this be a 
> reason for slowness?

They would, but only if the heads are off seeking through them.  If backups 
aren't active, the web interface doesn't have to access that much and should 
still be fairly fast.  If you are competing with a running backup it would slow 
down.

-- 
   Les Mikesell
    lesmikesell AT gmail DOT com


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