linux-kernel - RAID performance is not too well....

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Message-ID: <20070629140458.GA32107@bitwizard.nl>
Date:	Fri, 29 Jun 2007 16:04:59 +0200
From:	Rogier Wolff <R.E.Wolff@...Wizard.nl>
To:	linux-kernel@...r.kernel.org
Subject: RAID performance is not too well.... 

Hi,

I have an application that creates some 228 thousand files,
spread over about 4000 directories. Total is not more than 
1.3Gb.  (I'm not sure, and I don't care if it's 10% or 90% of
that number)

Anyway, I've loaded all of the 1.3Gb into the cache (the machine
has 8Gb of RAM), so that only writes need to take place. 

After a while the machine goes into a routine of writing
about 500 to 1000kbytes per second. 

Sync seems to take a long time: 

zebigbos:/recover7/bd4256_jense/tree> time sync 
0.004u 0.136s 5:44.66 0.0%      0+0k 0+0io 0pf+0w
zebigbos:/recover7/bd4256_jense/tree> 

The machine normally reads up to about 150 Mbytes per second without
trouble. 

I'm suspecting that the writes to the inodes and files all end
up "fragmented" such that reads to complete the RAID stripes 
need to be performed: 

Iostat shows: 

Device:            tps    kB_read/s    kB_wrtn/s    kB_read    kB_wrtn
sda              75.25       277.23       126.73        280        128
sdb              91.09       400.00       134.65        404        136
sdc              71.29       253.47        95.05        256         96
sdd             100.99       221.78       304.95        224        308

However, I would say that all those new files should be "clustered" 
such that the chances of writing a full stripe becomes reasonable. 
Moreover, clustering should, even with reading other parts of the
stripe result in a performance on the order of 10 to 50 times better. 

Raid block (stripe) size  is 64k.  (Next time I format a partition, 
I will chose 512k, causing the readperformance to increasae from 150Mb
per second to about 200Mb per second). 

	Roger. 

-- 
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