linux-ext4 - Re: [PATCH RFC 1/3] block: Add support for REQ_OP_ASSIGN

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Date:   Sat, 21 Dec 2019 13:54:50 -0500
From:   "Martin K. Petersen" <martin.petersen@...cle.com>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     "Martin K. Petersen" <martin.petersen@...cle.com>, axboe@...nel.dk,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-ext4@...r.kernel.org, tytso@....edu,
        adilger.kernel@...ger.ca, ming.lei@...hat.com, osandov@...com,
        jthumshirn@...e.de, minwoo.im.dev@...il.com, damien.lemoal@....com,
        andrea.parri@...rulasolutions.com, hare@...e.com, tj@...nel.org,
        ajay.joshi@....com, sagi@...mberg.me, dsterba@...e.com,
        chaitanya.kulkarni@....com, bvanassche@....org,
        dhowells@...hat.com, asml.silence@...il.com
Subject: Re: [PATCH RFC 1/3] block: Add support for REQ_OP_ASSIGN_RANGE operation


Kirill,

> One more thing to discuss. The new REQ_NOZERO flag won't be supported
> by many block devices (their number will be even less, than number of
> REQ_OP_WRITE_ZEROES supporters). Will this be a good thing, in case of
> we will be completing BLKDEV_ZERO_ALLOCATE bios in
> __blkdev_issue_write_zeroes() before splitting? I mean introduction of
> some flag in struct request_queue::limits.  Completion of them with
> -EOPNOTSUPP in block devices drivers looks suboptimal for me.

We already have the NOFALLBACK flag to let the user make that decision.

If that flag is not specified, and I receive an allocate request for a
SCSI device that does not support ANCHOR, my expectation would be that I
would do a regular write same.

If it's a filesystem that is the recipient of the operation and not a
SCSI device, how to react would depend on how the filesystem handles
unwritten extents, etc.

-- 
Martin K. Petersen	Oracle Linux Engineering