On Mon, 19 Aug 2013 10:10:18 +0900 taejin1999@xxxxxxxxxxxxxxxxx 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 > > -- > To unsubscribe from this list: send the line "unsubscribe linux-raid" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html
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