[lug] Robust storage

D. Stimits stimits at comcast.net
Tue May 3 13:26:32 MDT 2005

>>I've had extremely good luck with Linux software RAID over the same time
>>period.  Oh, add to it that the software RAID is well documented, if in no
>>other way than through the code.  A local guy lost his 3ware RAID array in
>>his home box last summer, it just got confused and he couldn't recover it.
>>He basically had the data, but needed to tell the controller, through
>>poking appropriate bits onto a new drive, to bring the array back up, but
>>3ware wouldn't cooperate with documenting those parts of the drive.
> Which flavor of disks are you using with Linux software RAID?  Any
> special hdparms you pass?

IMHO, if you want reliable disks, get SCSI. I like the modern Seagates. 
In all cases, keep the drive cooled, don't ever stack 2 drives next to 
each other without some sort of fan or metal cooling frame (most hot 
swap bays do this if they are not plastic frames). No hdparms needed.

>>Really, the battery backed up cache on the controller is for performance,
>>not safety.  It allows the controller to confirm data written to the disc
>>when it gets into the cache, instead of the normal case where it waits for
>>it to get on the disc.  The problem here is the added complexity may lead
>>to other subtle problems, as we saw with livejournal.
> The write cache is most definitely for performance.  However, the
> battery backup is for safety in case of power loss prior to the cache
> being written to disk.  In the case of livejournal, they had write
> cache enabled on both the RAID card and the drives - I can't recall a
> situation in which everyone lying to each other was a good thing. ;-)

Don't know if it actually works, but several disks will use the energy 
of the spinning disk to power a buffer flush if power dies, even without 
battery backup. I have no idea which disks try this, but it was 
fascinating reading about it a couple of years ago. Anyone know if this 
technology actually works or which disks try it?

D. Stimits, stimits AT comcast DOT net

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