RE: What is the exacting meaning of Striped_Cache_Size?

["Liang Yang" <multisyncfe991@hotmail.com>]

Dan,

Thanks for your reply. Still get two questions left.

Suppose I have a MD-RAID5 array which consists of 8 disks. 
1. Do we need to consider the chunk size of the RAID array when we set the
value of Striped_Cache_Size? For example, if the chunk size is changed from
64k to 256k, do we need to adjust the Striped_Cache_Size accordingly?

2. The performance improvement of large size I/O packets (128k, 256k) is
larger than small size I/O packets (512B, 1KB) when I change the
Striped_Cache_Size. How do you explain the difference here?

Liang

-----Original Message-----
From: dan.j.williams@gmail.com [mailto:dan.j.williams@gmail.com] On Behalf
Of Dan Williams
Sent: Sunday, January 21, 2007 11:07 PM
To: Liang Yang
Cc: linux-raid@vger.kernel.org
Subject: Re: What is the exacting meaning of Striped_Cache_Size?

On 1/21/07, Liang Yang <multisyncfe991@hotmail.com> wrote:
> Hello,
>
> I have tried to increase the Striped_Cache_Size from 256 (default for my
> MD-RAID5 array) to 8192, it does improve the MD-RAID5 Write performance
> which varies with the size of I/O packet.
>
> However, I'm still not very clean the meaning and the potential
performance
> impact of this Striped_Cache_Size? Is the unit for this parameter Byte or
> KiloByte?
>
> Could anyone here explain with a little bit more details?
>
stripe_cache_size is the number of stripes in the cache.  Each stripe
(strip) is composed one PAGE_SIZE block per disk.  If your page size
is 4k and you have 4 disks in your array then a stripe_cache_size of
256 is 4k * 4 * 256 = 4MB.

Increasing this number increases the chances that a write to the array
will not generate reads to satisfy the parity calculation.

> Thanks,
>
> Liang
>
Regards,

Dan