On Fri, Jun 09, 2017 at 10:06:26PM -0400, Sweet Tea Dorminy wrote: > >What is the xfs_info for this filesystem? > meta-data=/dev/mapper/tracer-vdo0 isize=256 agcount=4, > agsize=5242880 blks > = sectsz=512 attr=2, projid32bit=0 > data = bsize=1024 blocks=20971520, > imaxpct=25 > = sunit=0 swidth=0 blks > naming =version 2 bsize=4096 ascii-ci=0 > log =internal bsize=1024 blocks=10240, version=2 > = sectsz=512 sunit=0 blks, > lazy-count=1 > realtime =none extsz=4096 blocks=0, rtextents=0 > > > What granularity are these A and B regions (sectors or larger)? > A is 1k, B is 3k. > > >Are you running on some kind of special block device that reproduces this? > It's a device we are developing, > asynchronous, which we believe obeys FLUSH and FUA correctly but may > have missed some case; So Occam's Razor applies here.... > we > encountered this issue when testing an XFS filesystem on it, and other > filesystems appear to work fine (although obviously we could have > merely gotten lucky). XFS has quite sophisticated async IO dispatch and ordering mechanisms compared to other filesystems and so frequently exposes problems in the underlying storage layers that other filesystems don't exercise. > Currently, when a flush returns from the device, > we guarantee the data from all bios completed before the flush was > issued is stably on disk; Yup, that's according to Documentation/block/writeback_cache_control.txt, however.... > when a write+FUA bio returns from the > device, the data in that bio (only) is guaranteed to be stable on disk. The > device may, however, commit sequentially issued write+fua bios to disk in an > arbitrary order. .... XFS issues log writes with REQ_PREFLUSH|REQ_FUA. This means sequentially issued log writes have clearly specified ordering constraints. i.e. the preflush completion order requirements means that the block device must commit preflush+write+fua bios to stable storage in the exact order they were issued by the filesystem.... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
