Date: Tue, 22 Dec 2015 11:51:31 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Max Gurtovoy <maxg@mellanox.com> Cc: Sagi Grimberg <sagig@dev.mellanox.co.il>, Sagi Grimberg <sagig@mellanox.com>, freebsd-scsi@freebsd.org, Hans Petter Selasky <hanss@mellanox.com>, Oren Duer <oren@mellanox.com> Subject: Re: splitting iovecs to bios Message-ID: <20151222095131.GJ3625@kib.kiev.ua> In-Reply-To: <567919D3.6050004@mellanox.com> References: <56696E03.8050202@mellanox.com> <20151210150210.GY82577@kib.kiev.ua> <566D2C91.9050900@dev.mellanox.co.il> <567919D3.6050004@mellanox.com>
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On Tue, Dec 22, 2015 at 11:37:23AM +0200, Max Gurtovoy wrote: > Hi Konstantin, > > > > >> There might be indeed a reason, it could be that some drivers expect > >> blocking to be done by the userspace. The drivers could have some > >> restrictions on transfer sizes and atomicity of transfer, which would > >> be broken by the unconditional merge. I cannot give you an example > >> of such driver, known block-aware drivers like sa(4) only require the > >> bio size to be multiple of the basic block size. > > > > I'm surprised to learn that the generic access layer splits IO requests > > just because some block drivers cannot handle it. I'd expect that this > > sort of limitation would be communicated by the drivers in the form of > > device flag SI_NOMERGE. > > > >> OTOH, I see no issue with adding a SI_PHYSIOMERGE flag and doing the > >> merges for the driver in physio(), when unmapped request has consequtive > >> iov elements ending and starting at the page boundary. > > > > I'd say it should be the other way around, physio would always strive > > to append/merge iov elements but wouldn't in case the device does not > > support it. Moreover, some modern devices does not even require the page > > boundary alignment you mentioned. These devices can execute IO to/from > > any arbitrary scatter list of buffers. > > Do you know if this issue is on someone's plate ? > If it doesn't, maybe we can try to advance it and start implementing > some solutions. > As I said earlier and as Sagi mentioned, this feature can improve the > performance of modern devices. Sure, feel free to implement it. If you need a help, just ask.
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