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