Intel RAID 1 read performance

ochadd

ochadd

[H]ard|Gawd
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May 9, 2008
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I've always thought RAID 1 arrays were supposed to improve read speeds. Google seems to tell me a different story or at least doesn't clearly state that it does. Anyone have a link to RAID 1 vs. RAID 0 performance using the same controller and drives? Anything specific to the Intel SATA III 6gbps RAID 1 with SSDs?

Just for curiosities sake.
 
ochadd said:
Anyone have a link to RAID 1 vs. RAID 0 performance using the same controller and drives?
Click to expand...

The Windows-driver for Intel ICH10 doesn't distribute reads over the disks, while at least the Linux md-driver does. I don't know about the RAID-function in Windows 7/8 Professional+, maybe someone can jump in here. Read speed in systems that distribute requests should be appr. 1.8x of the write speed.
 
RAID1 is ~the speed of a single disk, as ALL the data is written on both HDs at the same time. RAID1 is "mirror".
RAID0 is much faster, as the data is striped above the (for example) 2 HDs, means 50% are written to one disk while the other 50% are written to the other one. RAID0 is "striped".
 
Why would reading from a mirror RAID (in this case, RAID 1) improve read speeds? The purpose of RAID 1 is for data redundancy, not performance increases. For that you have RAID 10, 5, 6, and other striped solutions involving either parity or mirroring.

But by all means, if RAID 1 does in fact increase performance then I'm all ears since it'll be the first time I've heard of it
 
Good raid controllers (and software) have, for some time now, been able to distribute reads across both drives in a mirror. Not really rocket science. Your data is located on 2 different drives - might as well read from both of them in parallel.
 
Xinmosni said:
Why would reading from a mirror RAID (in this case, RAID 1) improve read speeds? The purpose of RAID 1 is for data redundancy, not performance increases. For that you have RAID 10, 5, 6, and other striped solutions involving either parity or mirroring.

But by all means, if RAID 1 does in fact increase performance then I'm all ears since it'll be the first time I've heard of it
Click to expand...

Since all data exists on both disks I see no reason as to why read speeds from RAID 1 couldn't be nearly on par with RAID 0. If a request comes in to read byte A, B, C, and D then why not read A and B from disk 1 and read C and D from disk 2. If the second disk has failed read C and D from disk 1. My college professor, over a decade ago, told me RAID 1 increased read speed. I've never put a RAID 1 array in a situation where additional performance was needed. Server operating system drives is the only place I've ever used it.

To me a four disk RAID 10 could have the same read speed benefits as a four disk RAID 0 and write speeds close to a two disk RAID 0 due to the same principle. The data is on both disks so why not distribute the load.

Edit: Found this document from 3ware on another post here talking about some proprietary tech increasing RAID 1 read speed. I guess it just comes down to the controller logic if it's good enough to increase speeds. Not all RAID 1 arrays are created equal. http://www.3ware.com/products/pdf/twinstor.pdf
 
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RAID 1 will only increase read speeds if you use linux RAID 1 or zpool Mirrors.

With Windows or standard RAID controllers it will not work.

Linux and Solaris are just epic like that
 
The linux md driver spreads separate reads over all RAID1 disks. For a lot of concurrent random read accesses (i.e. multiple VM images) this is much better than RAID0 can possibly achieve. It will not improve singular large sequential reads, however. But those are usually fast enough.

It can of course do nothing for writes, but writes can be cached, while reads often cannot.
 
staticlag said:
RAID 1 will only increase read speeds if you use linux RAID 1 or zpool Mirrors.

With Windows or standard RAID controllers it will not work.

Linux and Solaris are just epic like that
Click to expand...
Many modern RAID cards will also read from both drives as well, adaptec seems to think this can be a problem if you're RAID-1ing a SSD and HHD together so they advertise in that situation they only read from the SSD.
 
DieTa said:
RAID1 is ~the speed of a single disk, as ALL the data is written on both HDs at the same time. RAID1 is "mirror".
RAID0 is much faster, as the data is striped above the (for example) 2 HDs, means 50% are written to one disk while the other 50% are written to the other one. RAID0 is "striped".
Click to expand...

depends on what you set the size to, not always %50 even down the middle.
 
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