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Re: [PATCH 10/17] block: Add new BDRV_ZERO_OPEN flag

From: Eric Blake
Subject: Re: [PATCH 10/17] block: Add new BDRV_ZERO_OPEN flag
Date: Tue, 4 Feb 2020 11:50:22 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1

On 2/4/20 11:34 AM, Max Reitz wrote:

+++ b/include/block/block.h
@@ -105,6 +105,16 @@ typedef enum {
       * for drivers that set .bdrv_co_truncate.
      BDRV_ZERO_TRUNCATE      = 0x2,
+    /*
+     * bdrv_known_zeroes() should include this bit if an image is
+     * known to read as all zeroes when first opened; this bit should
+     * not be relied on after any writes to the image.

Is there a good reason for this?  Because to me this screams like we are
going to check this flag without ensuring that the image has actually
not been written to yet.  So if it’s generally easy for drivers to stop
reporting this flag after a write, then maybe we should do so.

In patch 15 (implementing things in qcow2), I actually wrote the driver to return live results, rather than just open-time results, in part because writing the bit to persistent storage in qcow2 means that the bit must be accurate, without relying on the block layer's help.

But my pending NBD patch (not posted yet, but will be soon), the proposal I'm making for the NBD protocol itself is just open-time, not live, and so it would be more work than necessary to make the NBD driver report live results.

But it seems like it should be easy enough to also patch the block layer itself to guarantee that callers of bdrv_known_zeroes() cannot see this bit set if the block layer has been used in any non-zero transaction, by repeating the same logic as used in qcow2 to kill the bit (any write/write_compressed/bdrv_copy clear the bit, any trim clears the bit if the driver does not guarantee trim reads as zero, any truncate clears the bit if the driver does not guarantee truncate reads as zero, etc). Basically, the block layer would cache the results of .bdrv_known_zeroes during .bdrv_co_open, bdrv_co_pwrite() and friends would update that cache, and and bdrv_known_zeroes() would report the cached value rather than a fresh call to .bdrv_known_zeroes.

Are we worried enough about clients of this interface to make the block layer more robust? (From the maintenance standpoint, the more the block layer guarantees, the easier it is to write code that uses the block layer; but there is the counter-argument that making the block layer track whether an image has been modified means a [slight] penalty to every write request to update the boolean.)

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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