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Performance question: SAS 10K 6Gbps and SATA SSD 3Gbps caching?

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December 10, 2013 9:18:57 AM

Hi Guys,
I need to buy a storage, and I don't know which one of these options will give me more throughput.

I have a SAS disk at 10K rpm, with 6Gbps interface.

Using an SSD as caching will downgrade the performance if it's a SATA 3Gbps ?

HDD: longer access time due to physical media BUT faster bus speed
vs
SSD: Quicker access time BUT lower bus speed.

Any quick answer?

Thanks!!

More about : performance question sas 10k 6gbps sata ssd 3gbps caching

a c 105 G Storage
December 10, 2013 2:51:59 PM

what SSD are you getting? And is it actually a 3Gbps connection on the SSD or is that on the motherboard side that your plugging it into a 3Gbps SATA slot on motherboard? If its the second the SSD will STILL be faster than the SAS. If its a single drive it still probably only does like 150MBps where as the SSD if its a 6Gbps on a 3Gbps it will saturate the connection and get like the 250 range probably.
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December 11, 2013 6:10:20 AM

Hi,
Actually it's a 12 disk Storage.
The main idea is the following:
10x 900GB SAS 10k 6Gbps disk
2x 200GB SATA 3Gbps disks in RAID 1 acting as cache of the other 10.

Storage has SAS/SATA ports at 6Gbps.
Regards
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a c 105 G Storage
December 11, 2013 12:22:44 PM

Well since the main 10 are SAS, 10k sata 6 then using the sata 3 will be a bit of a down grade. Whole idea of Cashe is for it to be faster and to store the data while its being used, access, or written. If you go with some SSD's for Cashe they will probably work out pretty good even on Sata 3Gbps. If you use Hard Drives then I will probably be slower especially with some nice 900GB high density 10k SAS drives.
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a c 940 G Storage
December 11, 2013 4:12:36 PM

Normally you cant add sata drives to a controller running with sas drives on it. They use different signallinig levels (voltages). A Sas controller can run sata drives but not while its running sas drives. Go fetch your self a pair of enterprise ssd's.

As for the caching, even sata3gb would be a benefit, its the random file access that harddrives are slow with and SSD's are much better at. And randoms wouldn't saturate a sata2 bus
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a c 105 G Storage
December 11, 2013 8:01:40 PM

I've missed SATA and SAS on the same controller no problem. Its running them in the same Raid is where they are incompatible.
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December 12, 2013 4:52:14 AM

Regardless of SATA and SAS been able to run in the same controller at the same time,
About what popatim wrote:

"randoms wouldn't saturate a sata2 bus"

I guess it depends on the amount of randoms, so what's the limit sata 2 impose to random reads?

Thank you.
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a c 940 G Storage
December 12, 2013 9:45:05 AM

~280MB/s theoretical max speed.

I'll stand corrected on the sata/sas issue. Looking it up its on the expanders you shouldn't.
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