[CentOS] BIOS RAID0 and differences between disks

Thu Nov 5 03:47:07 UTC 2020
Valeri Galtsev <galtsev at kicp.uchicago.edu>

> On Nov 4, 2020, at 9:21 PM, John Pierce <jhn.pierce at gmail.com> wrote:
> is it RAID 0 (striped) or raid1 (mirrored) ??
> if you wrote on half of a raid0 stripe set, you basically trashed it.
> blocks are striped across both drives, so like 16k on the first disk, then
> 16k on the 2nd then 16k back on the first, repeat (replace 16k with
> whatever your raid stripe size is).
> if its a raid 1 mirror, then either disk by itself has the complete file
> system on it, so you should be able to remirror the changed disk onto the
> other drive.   you MUST do that re-mirror because your two disks are no
> longer identical, and reads will alternate between them, so some reads will
> get new data and others will get old data, which will be highly chaotic.

John, I figure, BIOS RAID is essentially software RAID (handled by Linux kernel’s md module, or whatever module's name is). I have a question then. If it were RAID-1, and one of the drives was mounted, would the kernel’s md (?) module recognize that the other drive is out of sync, - there should be timestamp when each of RAID members was last in sync/used. Right? Or I am mistaken?

If I understand correctly, OP has, or rather had (uh-huh), RAID-0.


PS. I thought I was wrong, but I was mistaken ;-)

> On Wed, Nov 4, 2020 at 6:18 PM H <agents at meddatainc.com> wrote:
>> My computer running CentOS 7 is configured to use BIOS RAID0 and has two
>> identical SSDs which are also encrypted. I had a crash the other day and
>> due to a bug in the operating system update, I am unable to boot the system
>> in RAID mode since dracut does not recognize the disks in grub. After
>> modifying the grub command line I am able to boot the system from one of
>> the harddisks after entering the LUKS password, seemingly without any
>> problems but am obviously not running in RAID0 mode. When I booted in
>> single-disk mode I am sure there were some new files created on the single
>> SSD the system sees but I fairly quickly shut it down until this can be
>> fixed.
>> My question is: once the operating system fix has been released and I can
>> once again boot in BIOS RAID0 mode and decrypting both SSDs (same password
>> entered only once of course), how will the BIOS RAID0 react? How will it
>> handle new files on one disk, altered timestamps etc.?
>> Thanks.
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> -- 
> -john r pierce
>  recycling used bits in santa cruz
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