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

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Date:   Thu, 19 Dec 2019 17:53:39 -0800
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     "Martin K. Petersen" <martin.petersen@...cle.com>
Cc:     Kirill Tkhai <ktkhai@...tuozzo.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

On Thu, Dec 19, 2019 at 05:37:47PM -0500, Martin K. Petersen wrote:
> 
> Kirill,
> 
> > Hm. BLKDEV_ZERO_NOUNMAP is used in __blkdev_issue_write_zeroes() only.
> > So, do I understand right that we should the below two?:
> >
> > 1) Introduce a new flag BLKDEV_ZERO_ALLOCATE for
> > blkdev_issue_write_zeroes().
> 
> > 2) Introduce a new flag REQ_NOZERO in enum req_opf.
> 
> Something like that. If zeroing is a problem for you.
> 
> Right now we offer the following semantics:
> 
> 	Deallocate, no zeroing (discard)
> 
> 	Optionally deallocate, zeroing (zeroout)
> 
> 	Allocate, zeroing (zeroout + NOUNMAP)
> 
> Some devices also implement a fourth option which would be:
> 
> 	Anchor: Allocate, no zeroing

What happens if you anchor and then try to read the results?  IO error?

(Yeah, I'm being lazy and not digging through SBC-3, sorry...)

--D

> > Won't this confuse a reader that we have blkdev_issue_write_zeroes(),
> > which does not write zeroes sometimes? Maybe we should rename
> > blkdev_issue_write_zeroes() in some more generic name?
> 
> Maybe. The naming is what it is for hysterical raisins and reflects how
> things are implemented in the storage protocols. I wouldn't worry too
> much about that. We can rename things if need be but we shouldn't plumb
> an essentially identical operation through the block stack just to
> expose a different name at the top.
> 
> -- 
> Martin K. Petersen	Oracle Linux Engineering