Bill Told me to report this to linux-raid. But Possibly
the raid code can't be blamed, because it is the filesystem
code that messes up.
----- Forwarded message from Bill Davidsen <[EMAIL PROTECTED]> -----
>
> From: Bill Davidsen <[EMAIL PROTECTED]>
> To: Rogier Wolff <[EMAIL PROTECTED]>
> CC: [EMAIL PROTECTED]
> Date: Thu, 05 Jul 2007 15:35:36 -0400
> Subject: Re: RAID performance is not too well....
> X-Spam-Status: No, score=3.0 required=5.0 tests=AWL,BAYES_99 autolearn=no
version=3.1.7-deb
> Rogier Wolff wrote:
> >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,
This should be chunk size. ^^^^^^^^
> >I will chose 512k, causing the readperformance to increasae from 150Mb
> >per second to about 200Mb per second).
> >
> I'm not sure what you mean by "Raid block," and a stripe size on 64k is
> improbably low. That sounds like a chunk size using common nomenclature.
OK. chunk size.
> You don't say what RAID level you are using, nor what filesystem type,
RAID 5.
> so it's really hard to give you any useful help, other than "send more
> information, use standard terms, send to the linux-raid list, not kernel."
>
> By "format a partition" do you mean "create an array?" Or ???
Ehh. Yes.
Roger.
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