On 2019/5/9 3:43 上午, Coly Li wrote:
> On 2019/5/8 11:58 下午, Thorsten Knabe wrote:
[snipped]
>> Hi Cody.
>>
>>> I cannot do this. Because this is real I/O issued to backing device, if
>>> it failed, it means something really wrong on backing device.
>>
>> I have not found a definitive answer or documentation what the
>> REQ_RAHEAD flag is actually used for. However in my understanding, after
>> reading a lot of kernel source, it is used as an indication, that the
>> bio read request is unimportant for proper operation and may be failed
>> by the block device driver returning BLK_STS_IOERR, if it is too
>> expensive or requires too many additional resources.
>>
>> At least the BTRFS and DRBD code do not take bio request IO errors that
>> are marked with the REQ_RAHEAD flag into account in their error
>> counters. Thus it is probably okay if such IO errors with the REQ_RAHEAD
>> flags set are not counted as errors by bcache too.
>>
>>>
>>> Hmm, If raid6 may returns different error code in bio->bi_status, then
>>> we can identify this is a failure caused by raid degrade, not a read
>>> hardware or link failure. But now I am not familiar with raid456 code,
>>> no idea how to change the md raid code (I assume you meant md raid6)...
>>
>> I my assumptions above regarding the REQ_RAHEAD flag are correct, then
>> the RAID code is correct, because restoring data from the parity
>> information is a relatively expensive operation for read-ahead data,
>> that is possibly never actually needed.
>
>
> Hi Thorsten,
>
> Thank you for the informative hint. I agree with your idea, it seems
> ignoring I/O error of REQ_RAHEAD bios does not hurt. Let me think how to
> fix it by your suggestion.
>
Hi Thorsten,
Could you please to test the attached patch ?
Thanks in advance.
--
Coly Li
From 355de6a3643959251bd90b0688df093f3c3d45b3 Mon Sep 17 00:00:00 2001
From: Coly Li <colyli@xxxxxxx>
Date: Mon, 13 May 2019 22:48:09 +0800
Subject: [PATCH] bcache: ignore read-ahead request failure on backing device
When md raid device (e.g. raid456) is used as backing device, read-ahead
requests on a degrading and recovering md raid device might be failured
immediately by md raid code, but indeed this md raid array can still be
read or write for normal I/O requests. Therefore such failed read-ahead
request are not real hardware failure. Further more, after degrading and
recovering accomplished, read-ahead requests will be handled by md raid
array again.
For such condition, I/O failures of read-ahead requests don't indicate
real health status (because normal I/O still be served), they should not
be counted into I/O error counter dc->io_errors.
Since there is no simple way to detect whether the backing divice is a
md raid device, this patch simply ignores I/O failures for read-ahead
bios on backing device, to avoid bogus backing device failure on a
degrading md raid array.
Suggested-by: Thorsten Knabe <linux@xxxxxxxxxxxxxxxxx>
Signed-off-by: Coly Li <colyli@xxxxxxx>
---
drivers/md/bcache/io.c | 12 ++++++++++++
1 file changed, 12 insertions(+)
diff --git a/drivers/md/bcache/io.c b/drivers/md/bcache/io.c
index c25097968319..d32e1681310c 100644
--- a/drivers/md/bcache/io.c
+++ b/drivers/md/bcache/io.c
@@ -58,6 +58,18 @@ void bch_count_backing_io_errors(struct cached_dev *dc, struct bio *bio)
WARN_ONCE(!dc, "NULL pointer of struct cached_dev");
+ /*
+ * Read-ahead requests on a degrading and recovering md raid
+ * (e.g. raid6) device might be failured immediately by md
+ * raid code, which is not a real hardware media failure. So
+ * we shouldn't count failed REQ_RAHEAD bio to dc->io_errors.
+ */
+ if (bio->bi_opf & REQ_RAHEAD) {
+ pr_warn("%s: Read-ahead I/O failed on backing device, ignore",
+ dc->backing_dev_name);
+ return;
+ }
+
errors = atomic_add_return(1, &dc->io_errors);
if (errors < dc->error_limit)
pr_err("%s: IO error on backing device, unrecoverable",
--
2.16.4
