On Tue, Feb 10, 2015 at 9:12 PM, John R Pierce <pierce@xxxxxxxxxxxx> wrote:
> On 2/10/2015 6:54 PM, Chris Murphy wrote:
>>
>> Why I avoid swap on md raid 1/10 is because of the swap caveats listed
>> under man 4 md. Is possible for a page in memory to change between the
>> writes to the two md devices such that the mirrors are in fact
>> different. The man page only suggests this makes scrub check results
>> unreliable, and that such a difference wouldn't be read (?) But I
>> don't understand this. So I just avoid it because I haven't thoroughly
>> tested it.
>
>
> if its possible for that to happen, then the whole swapping AND mdraid
> mechanisms in linux are badly broken.
I suggest not taking my word for it, and reading man (4) md, starting
with the paragraph "The most likely cause for an unexpected mismatch
on RAID1 or RAID10 occurs if a swap partition or swap file is stored
on the array" and including the following 4 paragraphs, and let me
know what you think it's saying. It made my eyebrows raise, but it
seems to be saying it's not actually resulting in corruption. The part
I don't understand is how a page change between the writes to two
(swap on) mirrors translates into unused swap and thus not a problem
that there's a (meaningful) mismatch between the two mirrors. If the
page write to disk happened at all, it seems like this is used rather
than not used swap.
For data (not swap), a related known issue for all raid 1 and 5 is a
series of common problems: regularly scheduled scrubs are necessary to
make sure bad sectors are identified and corrected, yet this isn't the
default behavior, it has to be configured; further, a reported
mismatch doesn't unambiguously tell us which copy is good (or bad),
it's merely reported that they're different. Ergo, regularly schedule
"checks" are a good idea, while "repair" is sort of a last resort
because it might cause the good copy to get overwritten.
This isn't broken. It's just the way it's designed. This is what
DIF/DIX (now PI), Btrfs and ZFS are meant to address. There's also
been some intermittent talk on linux-raid@ whether and how to get
checksums integrated there.
--
Chris Murphy
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