From: NeilBrown <neilb@suse.de>
To: taejin1999@davinci.snu.ac.kr
Cc: linux-raid@vger.kernel.org
Subject: Re: Does the chunk size of RAID5 really mean the "smallest atomic" mass of data?
Date: Mon, 19 Aug 2013 11:33:49 +1000 [thread overview]
Message-ID: <20130819113349.7497892e@notabene.brown> (raw)
In-Reply-To: <0d357591ae9c24e2e07dadd5ce7ee873.squirrel@davinci.snu.ac.kr>
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On Mon, 19 Aug 2013 10:10:18 +0900 taejin1999@davinci.snu.ac.kr wrote:
> I have counted the number of write requests from the raid disks which are
> implemented as virtual block devices.
> The result is
> ** the larger chunk size is, the more write requests are received per
> device.
That is peculiar. How do the sizes of the write requests compare?
>
> I was wondering what the "chunk size" means on RAID5 of mdadm.
> According to below site, chunk is the smallest atomic mass of data that
> can be written to the device.
That isn't correct.
> (https://raid.wiki.kernel.org/index.php/RAID_setup)
The text for chunksize for RAID5 is a little unclear, but I'm not sure how
you reached the above conclusion from it.
>
> The source code, however, seems to work differently.
> As I know, mdadm issues bio to each device in ops_run_io function.
It is "md" that issues bios etc. "md" is the kernel driver. "mdadm" is the
tool for ADMinistering MD arrays.
> In the code, the size of bio is set to be only STRIPE_SIZE.
> I think the device would get STRIPE_SIZE-sized data which is smaller than
> the chunk size.
Correct. Though the elevator might merge bios together.
>
> Is the chunk size not smallest data that can be written to the device?
It is not
> Or is there any procedure that merge the STRIPE_SIZE-sized data into
> chunk-sized data?
That depends on the lower level device that received the bios from md. It
might merge multiple bios into single requests.
>
> I am looking forward to your reply.
> Thank you.
>
> PS) Could you guess why more writes are issued for larger chunk size?
Maybe there are more read-modify-write cycles so the parity gets updates
multiple times.
NeilBrown
>
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prev parent reply other threads:[~2013-08-19 1:33 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-08-19 1:10 Does the chunk size of RAID5 really mean the "smallest atomic" mass of data? taejin1999
2013-08-19 1:33 ` NeilBrown [this message]
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