Hi Goswin, My understanding of chunk size is that, it is minimum unit of data that is used for read/write. Chunk size defines, amount of data written on one disk after breaking it in equal no of chunks. Chunk size = Stripe size / (disks in array - parity disks) so for raid 5, chunk_size = Stripe_Size / (total_disks -1) and for raid 6, chunk_size = Stripe_Size / total_disks -2) and parity is also written in terms of chunk size. Now considering a case where IO request came for complete 1 stripe, we need buffer, for 1 stripe, but since we allocated 1 page per chunk, instead of 64k buffer we have only 4k buffer, So how this is handled? Thanks & Regards, Kunal On Sat, May 2, 2009 at 3:11 AM, Goswin von Brederlow <goswin-v-b@xxxxxx> wrote: > Kunal Kushwaha <kunal.kushwaha@xxxxxxxxx> writes: > >> Hi Neil, >> >> Thanks for your reply. I am trying to find out problems that I could >> face later on. >> >> On Wed, Apr 29, 2009 at 7:15 AM, Neil Brown <neilb@xxxxxxx> wrote: >>> On Tuesday April 28, kunal.kushwaha@xxxxxxxxx wrote: >>>> Hi, >>>> >>>> I am trying to put Linux raid in Box with 256 MB of RAM. The kernel is >>>> compiled with non-swappable memory management option. I looked into >>>> raid5.c and found, it allocates one page for each chunk. I am using 5 >>>> disks for 64k chunk size. considering my kernel is within 30 MB. >>> >>> That isn't quite right. It is not 1 page per chunk. >>> raid5 maintains a stripe cache. Each entry in the cache has one page >>> per device, and there are 256 entries by default. >>> So for a 5-disk array, that is 5*256 == 1280 pages or 5MB (plus >>> overhead). >>> >> >> Sorry I missed to mention about stripe cache. Yes we will be using >> only 5MB if we >> consider page size is of 4k. >> >> I have one more doubt regarding this. How a page of 4k will be able to >> store data of >> 64k( my chunk size if of 64k) ? > > A chunk size of 64k does in no way mean all transactions are done in > 64k chunks. > > What effect does chunk size actually do have? Was it just the amount > of sequential data before the parity is rotated in a raid5/6. > > MfG > Goswin > -- Regards, Kunal Kushwaha -- 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
