[RndTbl] expert RAID question

Trevor Cordes trevor at tecnopolis.ca
Wed Apr 22 03:32:50 CDT 2015

I'm reading a book talking about RAID 5.  For writes (only), it says:

In the case when: "The write changes exactly one disk.  The parity context 
doesn't need to be recovered, since we can compute it just as if the disk 
had failed.  The write involves one write and one parity write.  
Performance is about 50% of a single disk."

("Parity context" in this book means the contents of that entire stripe; 
so saying it doesn't need to be recovered means that you don't need to 
read the other n-2 disks (+1 parity disk) that hold that stripe.)

I'm trying to figure how you "can compute it just as if the disk had 
failed".  This makes no sense to me.  I don't see how you could possibly 
get away with only 2 writes and no reads when changing just a single 
disk's block in a RAID 5 array.  What am I missing??

(This book does have errors, and I am wondering if this is one of them.)

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