Date: Sat, 9 Dec 2017 22:14:50 -0500 From: Mark Johnston <markj@freebsd.org> To: Warner Losh <imp@bsdimp.com> Cc: src-committers <src-committers@freebsd.org>, "svn-src-all@freebsd.org" <svn-src-all@freebsd.org>, "svn-src-head@freebsd.org" <svn-src-head@freebsd.org> Subject: Re: svn commit: r326731 - head/sys/ufs/ffs Message-ID: <20171210031450.GB15275@raichu> In-Reply-To: <CANCZdfoM0xhtFsajOQA2RB1FkX0y83z=vUUk72jcQNKqrdGkYQ@mail.gmail.com> References: <201712091544.vB9FiVUI096790@repo.freebsd.org> <a1d303b4-14b8-6a3d-3648-96a69bed7144@FreeBSD.org> <CANCZdfoM0xhtFsajOQA2RB1FkX0y83z=vUUk72jcQNKqrdGkYQ@mail.gmail.com>
next in thread | previous in thread | raw e-mail | index | archive | help
On Sat, Dec 09, 2017 at 07:36:59PM -0700, Warner Losh wrote: > On Sat, Dec 9, 2017 at 11:03 AM, Andriy Gapon <avg@freebsd.org> wrote: > > > On 09/12/2017 17:44, Mark Johnston wrote: > > > Some GEOMs do not appear to handle BIO_ORDERED correctly, meaning that > > the > > > barrier write may not work as intended. > > > There's a few places we send down a BIO_ORDERED BIO_FLUSH command > (see softdep_synchronize for one). Will those matter? Some classes have separate handling for BIO_FLUSH, so it depends. I think gmirror's handling is buggy independent of BIO_ORDERED: g_mirror_start() sends BIO_FLUSH commands directly to the mirrors, while reads and writes are queued for handling by the gmirror worker thread. So as far as I can tell, a BIO_WRITE which arrives at the gmirror provider before a BIO_FLUSH might be sent to the mirrors after that BIO_FLUSH. I would expect BIO_FLUSH to implicitly have something like release semantics, i.e., a BIO_FLUSH shouldn't be reordered with a BIO_WRITE that preceded it. But I might be misunderstanding. > As I've noted elsewhere: I'd really like to kill BIO_ORDERED since it has > too many icky effects (and BIO_FLUSH + BIO_ORDERED isn't guaranteed to do, > well, anything since it can turn into a NOP in a number of places. Plus > many of the implementations of BIO_ORDERED assume the drive is like SCSI > and you just set the right tag to make it 'ordered'. For ATA we issue a non > NCQ command, which is a full drain of outstanding commands, send this > command, then start them again which really shuts down the parallelism we > implemented NCQ for :(. We do similar for NVME which is even worse. There > we have multiple submission queues in the hardware. To simulated it, we do > a similar drain, but that's going to get in the way as we move to NUMA and > systems where we try to do the I/O entirely on one CPU (both submission and > completion) and ordered I/O is guaranteed lock contention. Independent of this, it doesn't really look like we have any way of handling write errors when dependencies are enforced using BIO_ORDERED. In the case of the babarrierwrite() consumer in FFS, what happens if the inode block write fails due to a transient error, but the following CG update succeeds?
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20171210031450.GB15275>