Re: RAID6 questions

[Goswin von Brederlow <goswin-v-b@web.de>]

Marek <mlf.conv@gmail.com> writes:

> Hi,
>
> I'm trying to build a RAID6 array out of 6x1TB disks, and would like
> to ask the following:
>
> 1. Is it possible to convert from 0.9 superblock to 1.x with mdadm
> 3.0? The reason is that most distributions ship with mdadm 2.6.x which
> seems to use 0.9 superblock by default. I wasn't able to find any info
> on mdadm 2.6.x using or switching to 1.x superblocks, so it seems that
> unless I'm using mdadm 3.0 which is practically unavailable, I'm stuck
> with 0.9.
>
> 2. Is it safe to upgrade to mdadm 3.x?
>
> 3. Is it possible to use 0xDA with 0.9 superblock and omit autodetect
> with mdadm 2.6.x? I couldn't find any information regarding this since
> most RAID related sources either still suggest 0xFD and
> autodetect(even with mdadm 3.0 by using -e 0.9 option) or they do not
> state which version of mdadm to use in case of 1.x superblocks. Since
> autodetect is deprecated, is there a safe way(without losing any data)
> to convert from autodetect + 0xFD in the future?

If you have raid build as module then the kernel does no
autodetect. Otherwise you can give some kernel commandline option, see
docs.

> 4. (probably a stupid question but..) Should an extended 0x05
> partition be ignored on RAID build? This is not directly related to
> mdadm, but many tutorials basically suggest to
> for i in `seq 1 x`; do mdadm --create (...) /dev/md$i /dev/sda$i
> /dev/sdb$i (...)
> It's not obvious in case one decides to partition the drives into many
> small partitions e.g. 1TB into 20x 50GB, in such case he gets 3
> primary partitions and one extended containing(or pointing to?) the
> remaining logical partitions, however the extended partition shows up
> as e.g. /dev/sda4, while the logical partitions appear as /dev/sda5,
> /dev/sda6 etc., so in the above mentioned case it would basically also
> try to create a RAID array from extended partitions.
> It would seem more logical to lay out the logical partitions as
> /dev/sda4l1 /dev/sda4l2 .... /dev/sda4l17 but udev doesn't seem to do
> that. Is it safe to ignore /dev/sdX4 and just create RAIDs out of
> /dev/sdX(1..3,5..20)?

Obviously you need to skip the extended partiton. I also see no reason
to create multiple raid6 over partitions on the same drive. Create one
big raid6 and use lvm or partitioning on that.

> 5. In case one decides for a partitioned approach - does mdadm kick
> out faulty partitions or whole drives? I have read several sources
> including some comments on slashdot that it's much better to split
> large drives into many small partitions, but noone clarified in
> detail.  A possible though unlikely scenario would be simultaneous
> failure of all hdds in the array:
>
>  md1 RAID6 sda1[_] sdb1[_] sdc1[U] sdd1[U] sde1[U] sdf1[U]
>  md2 RAID6 sda2[U] sdb2[_] sdc2[_] sdd2[U] sde2[U] sdf2[U]
>  md3 RAID6 sda3[U] sdb3[U] sdc3[_] sdd3[_] sde3[U] sdf3[U]
>  md4 RAID6 sda4[U] sdb4[U] sdc4[U] sdd4[_] sde4[_] sdf4[U]
>  md5 RAID6 sda5[U] sdb5[U] sdc5[U] sdd5[U] sde5[_] sdf5[_]
> (...)
>
> If mdadm kicks out faulty partitions only, but leaves the remaining
> part of drive going as long as it's able to read it, would it mean
> that even if every single hdd in the array failed somewhere (for
> example due to Reallocated_Sector_Ct), mdadm would keep the healthy
> partitions of that failed drive running, thus the entire system would
> be still running in degraded mode without loss of data?

The raid code kicks out a partition at a time when it gets errors. But
that means there must be an access to the partition for the kernel to
notice it does give errors first. So even if sda fails completly only
those drives you access will notice that and fail their sdaX.

In case of read errors the raid code also tries to restore a block
using the parity data and rewrite it so the drive can remap it to a
healthy sector.

> 6. Is it safe to have 20+ partitions for a RAID5,6 system? Most RAID
> related sources state that there's a limitation on number of
> partitions one can have on SATA drives(AFAIK 16), but i digged out
> some information about a recent patch which would remove this
> limitation and which according to some other source had also been
> accepted into mainline kernel, though I'm not sure about it.
> http://thread.gmane.org/gmane.linux.kernel/701825
> http://lwn.net/Articles/289927/

Should be 15 or unlimited. Look at the major/minor numbers of sda* and
sdb. After sda15 there is no space before sdb comes. So unless sda16
gets a dynamic major/minor it can't be accessed.

It certainly is safe. But it seems stupid as well.

> 7. Question about special metadata with X58 ICH10R controllers - since
> the 3.0 announcement states that the Intel Matrix metadata format used
> by recent Intel ICH controlers is also supported, I'd like to ask if
> there's some instructions available on how to use it and what benefits
> it would bring to the user.
>
> 8. Most RAID related sources seem to deal with rather simple scenarios
> such as RAID0 or RAID1. There are only a few brief examples avaliable
> on how to build RAID5 and none for RAID6. Does anyone know of any
> recent & decent RAID6 tutorial?

I don't see how the raid level is really relevant, esspecially between
raid5 and raid6. Raid6 just protects against 2 drives failing but
nothing changes in how to set it up or maintain it.

> thanks,
>
> Marek

MfG
        Goswin