Slow Sequential Read on RAID 1 ??? Why?

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XeoNoX

Junior Member
Feb 22, 2012
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I have a Intel 2500K Process and a 3TB RAID 1 on a 2 x SATA 3Gb/s Port on a Intel® Rapid Storage Technology RAID ARRAY. Im using the 3TB RAID 1 ARRAY for storage and im using a Crucial M4 128GB drive for my primary boot drive. My M4 drive operates normal good speeds. However on my RAID 1 ARRAY Ive noticed my Sequential Read speeds are slow (compared to what they should be), or at least they seem slow to me because im not sure if it is normal but my Sequential Read is around 75MB/s and the Sequential Write is around 140 MB/s which seems normal for the write but not for the READ. BEFORE i did any RAID the SEQ READ/WRITE were both around 120-140 for both drives. why is it that my READ speed is so slow after i did a RAID 1?? I've attached my CrystalDiskMark log below. My 3TB RAID 1 array are using a Seagate ST3000DM001 and a Seagate ST33000651AS



-----------------------------------------------------------------------
CrystalDiskMark 3.0.1 x64 (C) 2007-2010 hiyohiyo
Crystal Dew World : http://crystalmark.info/
-----------------------------------------------------------------------
* MB/s = 1,000,000 byte/s [SATA/300 = 300,000,000 byte/s]

Sequential Read : 73.481 MB/s
Sequential Write : 140.259 MB/s
Random Read 512KB : 49.253 MB/s
Random Write 512KB : 89.153 MB/s
Random Read 4KB (QD=1) : 0.728 MB/s [ 177.7 IOPS]
Random Write 4KB (QD=1) : 1.233 MB/s [ 300.9 IOPS]
Random Read 4KB (QD=32) : 3.247 MB/s [ 792.7 IOPS]
Random Write 4KB (QD=32) : 1.271 MB/s [ 310.3 IOPS]

Test : 500 MB [E: 33.6% (938.8/2794.4 GB)] (x2)
Date : 2012/02/22 8:51:38
OS : Windows 7 Ultimate Edition SP1 [6.1 Build 7601] (x64)
 
BTRY B 529th FA BN

BTRY B 529th FA BN

Lifer
Nov 26, 2005
15,188
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Just to get this question out of the way for everyone else, you do know Raid1 will only be as fast as the slowest drive or the speed of the matched drives, right?
 
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XeoNoX

Junior Member
Feb 22, 2012
8
0
61
BTRY B 529th FA BN said:
Just to get this question out of the way for everyone else, you do know Raid1 will only be as fast as the slowest drive or the speed of the matched drives, right?
Click to expand...


Yes, like i said in my original post before the raid BOTH drives were running around 120-140 average, after the raid now its 75 average
 
D

darkfalz

Member
Jul 29, 2007
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I suspect the Intel RAID 1 performance is slow because of:

When you do a write request, I don't think it writes identical data to both drives at once. I believe what it does is write to disk 0, then copy that data from disk 0 to disk 1. This makes writing incredibly slow.

I believe when it does a read request it reads from both drives, makes sure the data is the same and then that's the data you get. If the data is not the same, it overwites one with the other.

Now I'd expect this behaviour if the drives weren't fully mirrored yet, but I have no idea why this seems to continue even once in the Intel RST knows the drives are the same. Why it can't multiplex the writes is beyond me. As for reads, the data isn't striped but once it knows the drives are 1:1 exactly matched it should be able to independently grab queued data from either drive (thus speeding up especially random reads but also sequental reads by interleaving) but it doesn't seem to do any of this.

Bottom line is Intel RAID 1 is slow and next to useless. RAID 5 with write cache enabled on the other hand is plenty fast (I'm getting 300 MB/sec reads and 150 MB/sec writes on a 4 disk 5900 RPM drive array).
 
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