On Thu, Oct 29, 2015 at 12:23:12AM +0100, Andres Freund wrote: > Hi, > > On 2015-10-29 07:48:34 +1100, Dave Chinner wrote: > > > The idea of using SYNC_FILE_RANGE_WRITE beforehand is that > > > the fsync() will only have to do very little work. The language in > > > sync_file_range(2) doesn't inspire enough confidence for using it as an > > > actual integrity operation :/ > > > > So really you're trying to minimise the blocking/latency of fsync()? > > The blocking/latency of the fsync doesn't actually matter at all *for > this callsite*. It's called from a dedicated background process - if > it's slowed down by a couple seconds it doesn't matter much. > The problem is that if you have a couple gigabytes of dirty data being > fsync()ed at once, latency for concurrent reads and writes often goes > absolutely apeshit. And those concurrent reads and writes might > actually be latency sensitive. Right, but my point is with an async fsync/fdatasync you don't need this background process - you can just trickle out async fdatasync calls instead of trckling out calls to sync_file_range(). > By calling sync_file_range() over small ranges of pages shortly after > they've been written we make it unlikely (but still possible) that much > data has to be flushed at fsync() time. Right, but you still need the fsync call, whereas with a async fsync call you don't - when you gather the completion, no further action needs to be taken on that dirty range. > At the moment using fdatasync() instead of fsync() is a considerable > performance advantage... If I understand the above proposal correctly, > it'd allow specifying ranges, is that right? Well, the patch I sent doesn't do ranges, but it could easily be passed in as the iocb has offset/len parameters that are used by IOCB_CMD_PREAD/PWRITE. io_prep_fsync/io_fsync both memset the iocb to zero, so if we pass in a non-zero length, we could treat it as a ranged f(d)sync quite easily. > There'll be some concern about portability around this - issuing > sync_file_range() every now and then isn't particularly invasive. Using > aio might end up being that, not sure. It's still a non-portable/linux only solution, because it is using the linux native aio interface, not the glibc one... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
