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Old 09-08-2014, 06:01 AM   #1
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md raid1: combine slow read and fast write

I've assembled a raid1 using a SD-Card and a classic Hard Disk Drive (at least a small part of it).

(It's a small ARM home server, a second harddisk would be overkill. Most data is fine without a RAID so I added a SD-Card for the small part of data where I prefer redundancy.)

The SD Card is faster in read operations (commonly spreaded) but suffers from slow writes.

So I would like to use "write-mostly" with the hard disk and "write-behind" with the SD Card. (i.e. using the SD-Card for reading data but don't wait for the SD-Card to complete writing)

This combination seems to be impossible, because "write-behind" depends on "write-mostly". For me it seems not possible to use one with drive A and the other with drive B.

Any suggestions?
Old 09-09-2014, 04:32 PM   #2
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The combination you describe is good to speed up read-mostly applications like booting the OS. It is not good for write-intensive applications. I would just apply the write-mostly to the hard disk and be happy with your read speed.
Old 09-10-2014, 04:14 AM   #3
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For me read speed doesn't matter that much. Continuous transfer speeds not at all. Your link doesn't fit my case.

My case is a not heavily used imap storage and I think speed drawbacks are caused by waiting for flushing the data to disk. (i.e. sending a mail: store to "Drafts", fflush, store to "Queue", fflush, store to "Sent", fflush... ). Reading should be done by OS cache here!?

But... I think I have to investigate further into it, because it seems the SD-Card has only a high delay (>300ms) with the first write. Subsequent writes are faster (~40ms). Maybe some power savings mechanism.

Still the best would be to have "write-mostly" set with the HDD (to read from SD) and "write-behind" with the SD-Card (to only wait for flushing to the HDD), because it's not throughput which annoys, but the initial delay. A combination which is impossible, since it's not possible to use "write-behind" and "write-mostly" independently.

So maybe the best is to apply write-mostly and write-behind (both) to the SD-Card. So speed depends on the HDD only, which would be fine.


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