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Old 02-25-2010, 03:10 PM   #1
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ReadyNAS Duo: Under what circumstances is a resync really needed?

I've asked this over at the ReadyNAS forums a couple of time and have never gotten any responses at all. So I'm hoping that someone here actually has the answer.

It's not at all clear which specific forum this question belongs in, hence I'm posting it here. It's quite specific to the ReadyNAS units. Note that this is Linux-related to the extent that the ReadyNAS units run a variant of Debian Linux.

This is something of a technical question, and is mainly about the ReadyNAS Duo though it may apply to other units as well.

On the ReadyNAS Duo, a RAID resync synchronizes the entire contents of one disk to another. It's a slow operation. So it's not something to be done lightly. By default, it is done whenever the unit is shut down uncleanly (generally the result of a hang or power interruption). It's something that can be disabled, though, such that a resync would be done only when manually requested.

The authors of the software RAID driver in Linux (called "md") came up with a nice solution to the problem of slow RAID resyncs: the md driver allows you to assign a write intent bitmap to an md device, and where it's stored is configurable (the default is to store it on the md device in question, in the superblock). So when the array goes down unexpectedly (e.g., in a power outage), only the blocks that the md device was going to write will actually be resynced. It's sort of like a journal for the md device itself.

Now, the ReadyNAS Duo doesn't seem to use Linux md at all for implementing the RAID volume, but it does have an option to disable automatic resync in the event of an improper shutdown. And that leads to my question: how necessary is it to do a resync in such an event? If the Duo uses the same method as the Linux md driver, then obviously such a resync would rarely be necessary. But if it doesn't, then it would be the equivalent of using the md driver without a write-intent bitmap, and a resync would almost certainly be necessary in the event of an improper shutdown.

So: does the Duo (and maybe other units) use the equivalent of a write intent bitmap for RAID write operations?
Old 02-25-2010, 11:02 PM   #2
Registered: Dec 2003
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I think that there are two issues here. One involves the RAID disk replacement recovery, the other involves a partition integrity check.

The partitions are ext3 and an integrity check can take quite a while for large partitions. This is what is done when a system goes down unexpectedly. Do a 'man fsck' to see what that is about.

The RAID resync should only be needed if a drive fails and you have to replace it. That is, if you are using the default RAID setup which is for two redundant disks in the ReadyNAS Duo. Netgear uses their own RAID process, called X-RAID in order to gain features such as the ability to grow the size of disks in your system.

The question about a "write-intent bitmap" seems to be delving a bit deep into proprietary code which may be why you are having something of a problem finding an answer.

An ext3 file system check is slow for the same reason that the RAID resync is slow - both processes have to read the entire partition. That can take a while and the Duo processor slows things down some, too.


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