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

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Date:   Thu, 19 Dec 2019 21:22:36 -0500
From:   "Martin K. Petersen" <martin.petersen@...cle.com>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     "Martin K. Petersen" <martin.petersen@...cle.com>,
        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


Darrick,

[Anchor]

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

Depends on the device. Typically you get the same results as UNMAP.

Anchoring is essentially a way to preallocate blocks on a thinly
provisioned device. If you then run out of actual storage capacity you
won't get a write failure writing to those LBAs. It is intended to
protect block ranges that do not tolerate write failures (journals,
metadata).

-- 
Martin K. Petersen	Oracle Linux Engineering