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SATA II only transfers 25MB/sec to a raid 5?

Posted on 2008-10-29
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Running Vista Ultimate x64. XFX680iLT MB q6600, 4 gig ram, 3 WD green power SATA II drives in a raid 5.  I have a 1TB WD green power SATA II running through esata.  Here is my question.  When copying from my eSATA to the raid 5 i'm only getting about 20-25megabytes /sec.  Does that sound right.  If sata 2 is supposed to be 3Gb, shouldn't my copy rate be faster?  Any ideas?
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Question by:domsdabomb
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jdietrich earned 250 total points
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Yes, that seems a little low.  But remember, the 3 Gig is internal transfer rate, not external.  Also, Raid V is very poor for writes.  It spreads the data across the drives but due to the over head of parity (for recovery) it is not particularly ideal for writes.  Striping is the fastest as it uses all the drive controllers in tandem, however there is no fault tolerance.  Make sure you are using quality cables that are well shielded.  I would have thought your transfer rate would be closer to double.  Why not try the opposite, to see if it is the raid.  Copy from the raid to the esata.  Usually reads from raid V are better than writes....

JD
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20 - 25MB/sec isn't too bad. You can expect about 8 - 10MB/sec on a SATA drive for a typical mix of random and sequential data. And you're not going to like the write performance. RAID 5 suffers the RAID 5 write penalty - for every host write, the RAID controller must:
- Read the original data
- Read the original parity
- XOR the original parity with the new data to create the new parity
- Write the new parity
- Write the new data
So there are four disc operations for every host write. That can be a real performance killer if your RAID controller doesn't have write cache or the write cache fills up. Read operations on a RAID 5 set do not suffer the penalty and will usually give pretty good performance, but you should be aware that SATA drives sitting next to each other affect each other, and therefor affect the performance (see the article noted below for more information).

This is an excellent article that explains the difference between enterprise and personal storage, and why you won't git a gazillion MB/sec off SATA: http://www.seagate.com/content/docs/pdf/whitepaper/D2c_More_than_Interface_ATA_vs_SCSI_042003.pdf
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