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smart raid1 reads?

DISCUSSION Michael T. Babcock 2 participants 3 messages Aug 11, 2003 – Aug 13, 2003 1 message not in the archive MD RAID

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Michael T. Babcock

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Quoted 7 lines

In a raid0, of course, reads pull off every disk in parallel, thus performance scales with # of disks.

However, with raid1, it does not do this--it always reads from just one half of the mirror. Why doesn't it read from both halves of the mirror in parallel? I would expect it to, and would expect raid1 to perform similar to raid0 for reads.

I was looking for more information on why this is myself today. Its quite possible that the complexity is too great for anyone to be willing to try it yet. After all, if you want to read off of disk 0 and disk 1 at the same time, you need to be sure the data you're reading doesn't include any blocks waiting to be sync'd from the other drive.

I don't know what the in-memory structures look like for 'blocks waiting to be sync'd', but depending on their design, this could be easier said than done.

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Michael T. Babcock
C.T.O., FibreSpeed Ltd.
http://www.fibrespeed.net/~mbabcock
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Quoted 3 lines · Nicholas Leippe

I don't know about the internals either, but it's already issuing block writes to both drives in parallel, I don't see why synchronization is an issue. Block layers don't usually care about serialization--that is left to the filesystem and higher os levels, no? If so, I would think it shouldn't matter--md1 is in effect here the new block layer--merging two other block devices into one--nothing should be bypassing that layer for any blocks in the partition it's using.

Part of what bothers me is that even without parallelizing for one large read, if there are multiple queued reads from one RAID1 device that aren't nearby (see elevator code), then read the second set from the second drive, the third set from the first drive, and so on to minimize head movement.

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Michael T. Babcock
C.T.O., FibreSpeed Ltd.
http://www.fibrespeed.net/~mbabcock
#3 lore raw

Any mdadm based cgi perl script for webmin?

In reply to Michael T. Babcock

Hello,

I downloaded the latest WEBMIN, but its RAID tools are based on the RAIDTAB config and old tool.

I want to use new MDADM tool, but I am not an expert on PERL.

If anyone already modified it or know where some references are, May I get some new cgi perl program(or prototype) which use MDADM for webmin?

Those are under /usr/share/webmin/raid:

               create_raid.cgi,  raid_form.cgi, raid-lib.pl, ......

Thanks in advance,

Bo