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Re: SSDs with Postgresql?

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On Thu, 14 Apr 2011 11:46:12 +0200, Henry C. wrote:
On Thu, April 14, 2011 11:30, Leonardo Francalanci wrote:
have a look at


http://postgresql.1045698.n5.nabble.com/Intel-SSDs-that-may-not-suck-td426826
1.html



It looks like those are "safe" to use with a db, and aren't that expensive.

The new SSDs look great. From our experience, we trust SSDs (even MLC) far
more than mechanical hard drives.

I believe this perception that SSDs are less "safe" than failure-prone
mechanical hard drives will eventually change.

In the meantime, we've embraced them and the advantages are compelling.

h

One thing you should care about is such called write endurance - number of writes to one memory region before it will be destroyed - if your SSD driver do not have transparent allocation, then you may destroy it really fast, because write of each "block" will be in same memory segment, clog/xlog may be failed with 10k-100k writes. But if your SSD has transparent allocation, then internal controller will count your writes to given memory cell, and when lifetime of this cell will be at the end, it will "associate" block with different cell. With transparent allocation, You may sometimes do not fear if system uses journaling, you store logs there on any kind of often updatable data. You may calculate life time of your SSD with:
WritesToDestroyCells = "write_endurance"  * "disk_size"
AvgLifeTime = WritesToDestroyCells / writes_per_sec

Those are high numbers, even with simply disks as 10.000 * 60GB, means you need to send 600TB of data to one SSD (not completely true, as you can't send one byte, but full blocks) . Ofc, In order to extend life time of SSD you should provide file systems cache, or SSD with cache, as well turn off FS journaling.

Regards,
Radek

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