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Re: [Qemu-block] [Qemu-devel] [PATCH] blkdebug: Add support for latency

From: Marc Olson
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH] blkdebug: Add support for latency rules
Date: Mon, 27 Aug 2018 18:34:49 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

On 08/24/2018 09:11 AM, Eric Blake wrote:
On 08/24/2018 12:06 AM, Marc Olson via Qemu-devel wrote:
Allow rules to be created to inject latency into I/O operations.

Signed-off-by: Marc Olson <address@hidden>
  block/blkdebug.c        | 101 ++++++++++++++++++++++++++++++++++++++----------
  docs/devel/blkdebug.txt |  30 ++++++++++----
  2 files changed, 103 insertions(+), 28 deletions(-)

The commit message could usefully summarize some of the doc additions (so you can learn more about it without having to read the patch itself) - but the doc additions are appreciated.
I can be more verbose in the commit.

Missing: you should enhance an existing (or add a new) iotests usage of blkdebug to specify a latency operation. One possible test would be forcing a read to take longer than a write - then issue an aio read, a write immediately after, and verify that the write completes first and then the read (whether the read sees the old or the new data just written would then depend on whether the delay is acted on before or after blkdebug forwards the read on to the real block layer).

I'll add at least a simple test to the repo.

diff --git a/block/blkdebug.c b/block/blkdebug.c
index e216699..762e438 100644
--- a/block/blkdebug.c
+++ b/block/blkdebug.c
@@ -65,6 +65,7 @@ typedef struct BlkdebugSuspendedReq {
    enum {

Does any of this need to be accessible via QMP?  (If QMP can't already fine-tune what blkdebug can do, I'm not proposing that you have to make it do so - but if QMP can specify error injections, it should also be able to specify delays).  Even if it doesn't need to be in QMP, should we specify this enum and related types in our qapi/*.json files?
It does appear that blkdebug is in the qapi/*.json files. I've added the appropriate types, but I'm less familiar with the QMP incantations, and seem to have misconfigured my tests as they result in an uninitialized mutex when trying to do aio*. Using the existing tests as a guide, there's no global BlockBackend created, and so hmp_qemu_io() creates a local backend that is cleaned up before the aio completes. Removing the blk_unref() clears it up, but of course leaks memory. I don't see an API that creates the appropriate QMP BlockBackend for file backed devices.

@@ -123,6 +127,33 @@ static QemuOptsList inject_error_opts = {
  +static QemuOptsList delay_opts = {

One reason for suggesting that this be done with QAPI types is that QemuOpts is already proving to be painfully hard to extend, where in general we are trying to move towards using QAPI types rather than yet more QemuOpts munging.

@@ -182,16 +214,21 @@ static int add_rule(void *opaque, QemuOpts *opts, Error **errp)
          .state  = qemu_opt_get_number(opts, "state", 0),
  +    rule->once  = qemu_opt_get_bool(opts, "once", 0);
+    sector = qemu_opt_get_number(opts, "sector", -1);
+    rule->offset = sector == -1 ? -1 : sector * BDRV_SECTOR_SIZE;

I really wish we could spend time moving blkdebug to be byte-based instead of sector-based.
I'm not certain I see the benefit of moving to byte based. Storage devices are sector based. The biggest pain point with blkdebug is not that it's sector based, but that offsets are based on the backing file, and not as viewed by the guest. In order to properly use blkdebug with qcow2, for example, you have to do some gymnastics.

+++ b/docs/devel/blkdebug.txt
@@ -24,7 +24,7 @@ This way, all error paths can be tested to make sure they are correct.
  The blkdebug block driver takes a list of "rules" that tell the error injection
-engine when to fail an I/O request.
+engine when to fail (inject-error) or add latency to (delay) an I/O request.     Each I/O request is evaluated against the rules.  If a rule matches the request
  then its "action" is executed.
@@ -33,17 +33,25 @@ Rules can be placed in a configuration file; the configuration file   follows the same .ini-like format used by QEMU's -readconfig option, and
  each section of the file represents a rule.
  -The following configuration file defines a single rule:
+The following configuration file defines multiple rules:
      $ cat blkdebug.conf
    event = "read_aio"
    errno = "28"
  -This rule fails all aio read requests with ENOSPC (28).  Note that the errno
-value depends on the host.  On Linux, see
+  [delay]
+  event = "read_aio"
+  sector = "2048"
+  latency = "500000"
+The error rule fails all aio read requests with ENOSPC (28). Note that the
+errno value depends on the host.  On Linux, see
  /usr/include/asm-generic/errno-base.h for errno values.
  +The delay rule adds 500 ms of latency to a read I/O request containing sector

So in this example, you get a failure no matter what, but that one sector takes even longer to fail?
No, in the case of injection that overlaps, it takes the first rule. This is similar to the existing behavior--if two errors match, you don't know which you're getting. I like the suggestion, and I've reworked it a bit to delay first and then take any errors, but still take the first of each. I could see a follow on that looks for the most specific rule in either case (or the greatest delay in the latency case).

Overall, I like the idea.

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