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On my box running Ubuntu Server 11.10 I have a rather slow soft RAID6 array on four USB sticks (and no other means for persistent storage).

I want a faster filesystem on that box.

I'm OK if I'd lose some data in emergency situation, but I'd prefer the system and files to stay in consistent state in case of data loss.

I can set up a RAM-drive, but is there some more robust solution? (Like, maybe, configure my ext4 to use more RAM and sync less often... if that is a good idea.)

Update: Please assume that the 4xUSB sticks, RAID6 and ext4 are constants as far as this question is concerned. I can tune RAID6 and ext4 settings, I can set up something on top of that (or on a side — like RAM-drive), but no lower-level changes.

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In a 4 disk array, there is no reason to use RAID 6 over 10. I you want better performance, use a better RAID layout. –  MDMarra Mar 10 '12 at 19:35
    
This isn't constructive... Recipe for disaster. –  ewwhite Mar 10 '12 at 19:48
    
"Disaster" is relative. –  Alexander Gladysh Mar 10 '12 at 20:12
    
To clarify: loss or corruption of whole system on that box is not a disaster. (Inconvenience — yes, disaster — no.) –  Alexander Gladysh Mar 10 '12 at 20:22

1 Answer 1

up vote 4 down vote accepted

RAID5/6 are probably a bad idea on flash. Lots of additional reads and writes for every operation.

Rebuild as RAID10?

USB sticks are also not generally that fast. You might be better served by two USB HDDs. (if not ssds)

Since you oddly can't make any changes to the RAID, you might look at flashcache. (it let's you front one block device with another faster one and acts as a simple L2 cache) The typical use case is a SSD in front of a HDD, but a Ramdisk in front of your USBFlash may help you out here.

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Sorry, but you're answering the wrong question. Slow underlying FS is given, I need something on top of it. –  Alexander Gladysh Mar 10 '12 at 19:19
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Ok. You weren't clear that the root cause of the slowness was unavoidable. (my engine is small and my tires are flat, but I can't drive fast). Try flashcache. You could front the raid6 with a ram disk. –  Joel K Mar 10 '12 at 19:31
    
Exactly my situation. Thanks, I'll try flashcache — please update your answer. –  Alexander Gladysh Mar 10 '12 at 20:01
    
BTW, is the RAID6->RAID10 rebuild even possible? –  Alexander Gladysh Mar 10 '12 at 20:12
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Alexander: A RAID6->RAID10 'rebuild' should be possible. You have two spare drives. They could be forced offline. Build a half of the RAID10 on those. Reboot using the newly build half RAID (+test!). Then dump the remaining 2 drives and build a full RAID 10. I also want to stress Joels answer. Lots of writes to flash based storage is not a great idea. You wrote that you can't change that. Ok. that might be a given. But make sure you have good backups and that you get a warning when one of the four pendrives starts to fail. –  Hennes Mar 10 '12 at 20:28

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