From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Liang Yang" Subject: RE: What is the exacting meaning of Striped_Cache_Size? Date: Sun, 21 Jan 2007 23:22:24 -0700 Message-ID: References: Mime-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit Return-path: Message-ID: <000d01c73ded$ae7894f0$0401a8c0@bellevue> In-Reply-To: Sender: linux-raid-owner@vger.kernel.org To: 'Dan Williams' Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids 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 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