New: hv_storvsc - write cache enabled by default and cannot be disabled
https://bugzilla.kernel.org/show_bug.cgi?id=42939
Summary: hv_storvsc - write cache enabled by default and cannot
be disabled
Product: SCSI Drivers
Version: 2.5
Platform: All
OS/Version: Linux
Tree: Mainline
Status: NEW
Severity: normal
Priority: P1
Component: Other
AssignedTo: [email protected]
ReportedBy: [email protected]
Regression: No
dmesg output:
[ 8.080401] sd 0:0:0:0: [sda] Write cache: enabled, read cache: enabled,
supports DPO and FUA
Question: how safe is my (database) data in the case of a host machine crash or a VM crash?
There is no battery-backed writecache installed in the host, just some cheap and slow SATA disks.
Another curiosity occurs when running the fio iometer benchmark on the same
machine: it gives me 5.000 to 6.000 I/O random access operations per second when using the para-virt hv_storvsc driver, but only the expected 200 I/O ops when using ata_piix via emulation.The kernel in question is version 3.2.11.
There doesn't seem to be any way to disable the para-virt device's write-cache.
Kernel version is mainline 3.2.11.
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[Bug 42939] hv_storvsc - write cache enabled by default and cannot be disabled
https://bugzilla.kernel.org/show_bug.cgi?id=42939
Mark <[email protected]> changed:
What |Removed |Added
----------------------------------------------------------------------------
Kernel Version| |3.2.11
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[Bug 42939] hv_storvsc - write cache enabled by default and cannot be disabled
In reply to [email protected]https://bugzilla.kernel.org/show_bug.cgi?id=42939
--- Comment #1 from Mark <[email protected]> 2012-03-19 19:33:29 --- Additional information:
trying to disable the writecache leads to the partition on the SCSI device getting remounted read-only due to device errors.
Additional information (2):
I get 5.000 to 6.000 random reads per second even when manually creating the fiometer file using random data from /dev/urandom (ie. fio doesn't access zeroes in that case).
Additional information (3):
The host doesn't seem to cache anything: "dd iflag=direct if=/dev/sda bs=1M of=/dev/null count=10" always and repeatedly gives me around 30-60 MB/s.
Additional information (4):
I did a simple MySQL InnoDB ACID transaction test where subsequent transactions are done via a remote mysql client as fast as possible. I crashed the host hardware and checked if all transactions reported as completed had been committed to disk: no obvious problems there -- seemingly the reported writecache has no writecache-effects at all...??
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[Bug 42939] hv_storvsc - write cache enabled by default and cannot be disabled
In reply to [email protected]https://bugzilla.kernel.org/show_bug.cgi?id=42939
--- Comment #2 from Mark <[email protected]> 2012-03-19 19:36:47 --- Additional information (5):
I'm getting 1.000 subsequent (serialized and small -- integer inserts) transactions done (locally) within 2.2 seconds -- with InnoDB ACID transactions (ie. full data transaction sync enabled). Isn't that a bit too fast for cheap SATA disks in a RAID10 config?
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