Re: Basic RAID5/6 reshape question

[Brad Campbell <brad@wasp.net.au>]

Ryan Wagoner wrote:
> It takes 3 hours to resync my 3 x 1 TB drive RAID 5 array at a rate of
> 85MB/s. It shouldn't take 20 hours to write 3 x 1 TB drives. How fast
> is it progressing? You might consider a PCI-X or PCI Express SATA or
> SAS controller card. I have my drives connected to an Supermicro LSI
> 1068 PCI Express SAS card, which performs great.
>

I'm using 2 x Marvell 7042 based PCI-E x4 controllers and a SIL 3xxx (I don't recall but its PCI-E 
x1 and 2 ports). Bandwidth is not an issue.

A resync appears to be a completely different kettle of fish to a reshape. With re-shape it appears 
to write status data to the raid superblocks periodically. This appears to thrash the heads from the 
current position to the end of the disk and back. At the start of the reshape I was running about 
15-20MB/s to each drive. As the reshape progressed to the end of the disks I was looking at closer 
to 40MB/s. So obviously the final figure was less than the initial 20 hours, but a hell of a lot 
slower than a straight resync.

This blind stab at reshape behaviour is absolutely pure conjecture based on a gross guess at what 
was happening, however it does seem to bear out scrutiny.

> The other option would be to create 2 RAID 5 arrays of 5 drives each.
> Create the first array and use LVM with the filesystem of your choice.
> Add the second array later and expand the LVM volume.

Mmm thanks, but no.. to use a car analogy, "I'd like something I can fix with a screwdriver and a 
hammer". LVM is overcomplicating things way past what I require. In addition, I really like RAID-6. 
Over the last 4 years with my 15 drive RAID-6 I have had two double drive failures. Both times one 
drive died in a not so nice fashion, and one developed a grown defect during the rebuild. Raid-5 
just does not cut the mustard with me anymore.


Regards,
Brad
-- 
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