On Thu, 2014-01-23 at 13:27 -0800, Joel Becker wrote: +AD4- On Wed, Jan 22, 2014 at 10:47:01AM -0800, James Bottomley wrote: +AD4- +AD4- On Wed, 2014-01-22 at 18:37 +-0000, Chris Mason wrote: +AD4- +AD4- +AD4- On Wed, 2014-01-22 at 10:13 -0800, James Bottomley wrote: +AD4- +AD4- +AD4- +AD4- On Wed, 2014-01-22 at 18:02 +-0000, Chris Mason wrote: +AD4- +AD4- +AFs-agreement cut because it's boring for the reader+AF0- +AD4- +AD4- +AD4- +AD4- Realistically, if you look at what the I/O schedulers output on a +AD4- +AD4- +AD4- +AD4- standard (spinning rust) workload, it's mostly large transfers. +AD4- +AD4- +AD4- +AD4- Obviously these are misalgned at the ends, but we can fix some of that +AD4- +AD4- +AD4- +AD4- in the scheduler. Particularly if the FS helps us with layout. My +AD4- +AD4- +AD4- +AD4- instinct tells me that we can fix 99+ACU- of this with layout on the FS +- io +AD4- +AD4- +AD4- +AD4- schedulers ... the remaining 1+ACU- goes to the drive as needing to do RMW +AD4- +AD4- +AD4- +AD4- in the device, but the net impact to our throughput shouldn't be that +AD4- +AD4- +AD4- +AD4- great. +AD4- +AD4- +AD4- +AD4- +AD4- +AD4- There are a few workloads where the VM and the FS would team up to make +AD4- +AD4- +AD4- this fairly miserable +AD4- +AD4- +AD4- +AD4- +AD4- +AD4- Small files. Delayed allocation fixes a lot of this, but the VM doesn't +AD4- +AD4- +AD4- realize that fileA, fileB, fileC, and fileD all need to be written at +AD4- +AD4- +AD4- the same time to avoid RMW. Btrfs and MD have setup plugging callbacks +AD4- +AD4- +AD4- to accumulate full stripes as much as possible, but it still hurts. +AD4- +AD4- +AD4- +AD4- +AD4- +AD4- Metadata. These writes are very latency sensitive and we'll gain a lot +AD4- +AD4- +AD4- if the FS is explicitly trying to build full sector IOs. +AD4- +AD4- +AD4- +AD4- OK, so these two cases I buy ... the question is can we do something +AD4- +AD4- about them today without increasing the block size? +AD4- +AD4- +AD4- +AD4- The metadata problem, in particular, might be block independent: we +AD4- +AD4- still have a lot of small chunks to write out at fractured locations. +AD4- +AD4- With a large block size, the FS knows it's been bad and can expect the +AD4- +AD4- rolled up newspaper, but it's not clear what it could do about it. +AD4- +AD4- +AD4- +AD4- The small files issue looks like something we should be tackling today +AD4- +AD4- since writing out adjacent files would actually help us get bigger +AD4- +AD4- transfers. +AD4- +AD4- ocfs2 can actually take significant advantage here, because we store +AD4- small file data in-inode. This would grow our in-inode size from +AH4-3K to +AD4- +AH4-15K or +AH4-63K. We'd actually have to do more work to start putting more +AD4- than one inode in a block (thought that would be a promising avenue too +AD4- once the coordination is solved generically. Btrfs already defaults to 16K metadata and can go as high as 64k. The part we don't do is multi-page sectors for data blocks. I'd tend to leverage the read/modify/write engine from the raid code for that. -chris -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
