Re: consistency detect

On Sunday October 10, mingz@ele.uri.edu wrote:

I have a question on RAID error detect. hope somebody can help me to
find it out. thanks.

take raid1 as an example, if one disk fail, raid 1 can detect the data
on disk is compromised and then reconstruct it using a spare disk. this
is straight forward.

but if one request comes to raid1 and raid1 sends requests to both
disks, at this time, system reboots because power outage, system
crashes, or any other reason. then after system reboots, how raid 1
detects which disk has consistent data? since before reboot, anything
can happen, data may in disk1 but not in disk2, or in disk2 but not in
disk1, or not in both disks, or already on both disks.

When you have a computer with a single drive, and it crashes due to
power outage or similar, and it was in the process of writing data out
to disk, the contents of those blocks that were being written is
undefined.  It might have the old data.  It might have the new data.
If there are multiple blocks being written, some might be "old", some
might be "new".

Exactly the same is true with RAID1.  There is "right" value for any
block that was in the process of being written.
RAID1 simply chooses a value and makes sure that it is the same on
both (all) drives.  It arbitrarily chooses the "first" drive in the
array and copies that onto the rest.

how raid1 or other raid code deal with this?

This sort of inconsistency is not really something for RAID to deal
with.  It is something for the filesystem or application to deal
with.  Possibly via journalling.  Possibly via 'fsck'.

NeilBrown

ming


-- 
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| Ming Zhang, PhD. Student
| Dept. of Electrical & Computer Engineering
| College of Engineering
| University of Rhode Island
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| e-mail: mingz at ele.uri.edu
| Tel. (401) 874-2293 
| Fax. (401) 782-6422
| http://www.ele.uri.edu/~mingz/
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