On 08/30/2009 08:55 AM, david@xxxxxxx wrote:
On Sun, 30 Aug 2009, Pavel Machek wrote:
From: Theodore Tso <tytso@xxxxxxx>
To use your ABS brakes analogy, just becase it's not safe to rely on
ABS brakes if the "check brakes" light is on, that doesn't justify
writing something alarmist which claims that ABS brakes don't work
100% of the time, don't use ABS brakes, they're broken!!!!
If it only was this simple. We don't have 'check brakes' (aka
'journalling ineffective') warning light. If we had that, I would not
have problem.
It is rather that your ABS brakes are ineffective if 'check engine'
(RAID degraded) is lit. And yes, running with 'check engine' for
extended periods may be bad idea, but I know people that do
that... and I still hope their brakes work (and believe they should
have won suit for damages should their ABS brakes fail).
the 'RAID degraded' warning says that _anything_ you put on that block
device is at risk. it doesn't matter if you are using a filesystem
with a journal, one without, or using the raw device directly.
David Lang
The easiest way to lose your data in Linux - with RAID, without RAID,
S-ATA or SAS - is to run with the write cache enabled.
If you compare the size of even a large RAID stripe it will be measured
in KB and as this thread has mentioned already, you stand to have damage
to just one stripe (or even just a disk sector or two).
If you lose power with the write caches enabled on that same 5 drive
RAID set, you could lose as much as 5 * 32MB of freshly written data on
a power loss (16-32MB write caches are common on s-ata disks these days).
For MD5 (and MD6), you really must run with the write cache disabled
until we get barriers to work for those configurations.
It would be interesting for Pavel to retest with the write cache
enabled/disabled on his power loss scenarios with multi-drive RAID.
Regards,
Ric
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