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Re: [Qemu-devel] [PATCHv3] block: optimize zero writes with bdrv_write_z

From: Peter Lieven
Subject: Re: [Qemu-devel] [PATCHv3] block: optimize zero writes with bdrv_write_zeroes
Date: Wed, 07 May 2014 16:26:56 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0

On 07.05.2014 04:52, Eric Blake wrote:
On 05/06/2014 06:23 PM, Peter Lieven wrote:
this patch tries to optimize zero write requests
by automatically using bdrv_write_zeroes if it is
supported by the format.

This significantly speeds up file system initialization and
should speed zero write test used to test backend storage

Signed-off-by: Peter Lieven <address@hidden>
v2->v3: - moved parameter parsing to blockdev_init
         - added per device detect_zeroes status to
           hmp (info block -v) and qmp (query-block) [Eric]
         - added support to enable detect-zeroes also
           for hot added devices [Eric]
         - added missing entry to qemu_common_drive_opts
         - fixed description of qemu_iovec_is_zero [Fam]

+static BdrvDetectZeroes parse_detect_zeroes(const char *buf, Error **errp)
+    if (!buf || !strcmp(buf, "off")) {
+        return BDRV_DETECT_ZEROES_OFF;
+    } else if (!strcmp(buf, "on")) {
+        return BDRV_DETECT_ZEROES_ON;
+    } else if (!strcmp(buf, "unmap")) {
+    } else {
+        error_setg(errp, "invalid value for detect-zeroes: %s",
+                   buf);
+    }
Isn't there QAPI generated code that you can use instead of open-coding
this conversion between string and enum values?

Actually I have no idea. As you pointed out in the qapi patch I sent
it was quite hard for me to crawl through the whole stuff as one who is not
familiar with it. Can somebody advise here? Anyhow, I wonder
how this would work since qapi doesn't know the C Macros.

+++ b/qapi-schema.json
@@ -937,6 +937,8 @@
  # @encryption_key_missing: true if the backing device is encrypted but an
  #                          valid encryption key is missing
+# @detect_zeroes: detect and optimize zero writes (Since 2.1)
  # @bps: total throughput limit in bytes per second is specified
  # @bps_rd: read throughput limit in bytes per second is specified
@@ -972,6 +974,7 @@
    'data': { 'file': 'str', '*node-name': 'str', 'ro': 'bool', 'drv': 'str',
              '*backing_file': 'str', 'backing_file_depth': 'int',
              'encrypted': 'bool', 'encryption_key_missing': 'bool',
+            'detect_zeroes': 'str',
For new additions, we try to prefer dash over underscore.  Eww - we've
already been inconsistent in this struct, with backing_file vs. node-name.

Maybe s/detect_zeroes/detect-zeroes/.  I obviously can't argue too
strongly in light of the rest of the struct in isolation.  However, you
DID use detect-zeroes on the input side later in the patch, so being
consistent between the two sites would be nice (given that node-name was
named consistently).

I tried to be consistent here. All "setters" use a dash while all "getters"
have an underscore. Look e.g. at all the I/O thortteling parameters.
One reason might be that a dash is not allowed as a member name in a struct.

From this perspective the only inconsistent one is node-name.

On the other hand, I _can_ argue strongly that open-coding this as 'str'
is wrong.  You already added an enum type, so use it:

'detect_zeroes': 'BlockdevDetectZeroesOptions',

(hmm, in this context, it's not really an option, so maybe there is some
other name we can bikeshed about; but beyond 'BlockdevDetectZeroes', I
don't have any other good ideas)

I tried to, however it did not compile for me when I tried this.

zero is one of those odd words with two different pluralized spellings
both in common use.  Code base currently has a 1:2 ratio between 'zeros'
vs. 'zeroes', so I guess you're okay; on the other hand, 'man qemu-img'
documents that 'convert -S' detects "zeros".

The reason I choosed zeroEs is that the function in the background
is named bdrv_write_zeroEs.

              'bps': 'int', 'bps_rd': 'int', 'bps_wr': 'int',
              'iops': 'int', 'iops_rd': 'int', 'iops_wr': 'int',
              'image': 'ImageInfo',
@@ -4250,6 +4253,20 @@
    'data': [ 'ignore', 'unmap' ] }
+# @BlockdevDetectZeroesOptions
+# Selects the operation mode for zero write detection.
s/Selects/Describes/ since you are also going to use this enum on the
output field.


+# @off:      Disabled
+# @on:       Enabled
Maybe more details?  Seeing this doesn't tell me the tradeoffs involved
in tweaking the parameter (one is faster, while the other uses less
storage resources - so maybe mention those benefits).  I see the
documentation later on for the command line, so maybe repeating some of
that here would help someone going by just the QMP interface.

Will do.


+# @unmap:    Enabled and even try to unmap blocks if possible
+# Since: 2.1
+{ 'enum': 'BlockdevDetectZeroesOptions',
+  'data': [ 'off', 'on', 'unmap' ] }
  # @BlockdevAioOptions
  # Selects the AIO backend to handle I/O requests
@@ -4327,7 +4346,8 @@
              '*aio': 'BlockdevAioOptions',
              '*rerror': 'BlockdevOnError',
              '*werror': 'BlockdevOnError',
-            '*read-only': 'bool' } }
+            '*read-only': 'bool',
+            '*detect-zeroes': 'BlockdevDetectZeroesOptions' } }
This part is fine.

  @var{copy-on-read} is "on" or "off" and enables whether to copy read backing
  file sectors into the image file.
address@hidden address@hidden
address@hidden is "off", "on" or "unmap" and enables the automatic
+conversion of plain zero writes by the OS to driver specific optimized
+zero write commands. If "unmap" is chosen and @var{discard} is "on"
+a zero write may even be converted to an UNMAP operation.
This looks good.

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