well ... me again Following your advice.... I added a deadline for every WRITE stripe head when it is created. in raid5_activate_delayed i checked if deadline is expired and if not i am setting the sh to prereadactive mode as . This small fix ( and in few other places in the code) reduced the amount of reads to zero with dd but with no improvement to throghput. But with random access to the raid ( buffers are aligned by the stripe width and with the size of stripe width ) there is an improvement of at least 20 % . Problem is that a user must know what he is doing else there would be a reduction in performance if deadline line it too long (say 100 ms). raz On 7/3/06, Neil Brown <neilb@xxxxxxx> wrote:
On Sunday July 2, raziebe@xxxxxxxxx wrote: > Neil hello. > > I have been looking at the raid5 code trying to understand why writes > performance is so poor. raid5 write performance is expected to be poor, as you often need to pre-read data or parity before the write can be issued. > If I am not mistaken here, It seems that you issue a write in size of > one page an no more no matter what buffer size I am using . I doubt the small write size would contribute more than a couple of percent to the speed issue. Scheduling (when to write, when to pre-read, when to wait a moment) is probably much more important. > > 1. Is this page is directed only to parity disk ? No. All drives are written with one page units. Each request is divided into one-page chunks, these one page chunks are gathered - where possible - into strips, and the strips are handled as units (Where a strip is like a stripe, only 1 page wide rather then one chunk wide - if that makes sense). > 2. How can i increase the write throughout ? Look at scheduling patterns - what order are the blocks getting written, do we pre-read when we don't need to, things like that. The current code tries to do the right thing, and it certainly has been worse in the past, but I wouldn't be surprised if it could still be improved. NeilBrown
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