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From: Greg Smith <greg@2ndquadrant.com>
To: Yeb Havinga <yebhavinga@gmail.com>
Cc: Florian Weimer <fweimer@bfk.de>
Cc: Andy <angelflow@yahoo.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: BBU still needed with SSD?
Date: Tue, 19 Jul 2011 10:29:43 -0400
Message-ID: <4E2594D7.7000001@2ndquadrant.com> (raw)
In-Reply-To: <4E257587.3040704@gmail.com>
References: <1310953399.25726.YahooMailClassic@web111304.mail.gq1.yahoo.com>
	<4E23E72B.2070508@gmail.com>
	<82vcuyd7p5.fsf@mid.bfk.de>
	<4E255DF9.20005@gmail.com>
	<82y5zubl90.fsf@mid.bfk.de>
	<4E2569B0.2050307@gmail.com>
	<82livubix4.fsf@mid.bfk.de>
	<4E257587.3040704@gmail.com>

Yeb Havinga wrote:
> So for the Intels it's probably also lifetime writes in GB but you'd 
> have to check with an Intel smart values reader to be absolutely sure.

With my 320 series drive, the LBA units are pretty clearly 32MB each.  
Watch this:

root@toy:/ssd/data# smartctl --version
smartctl 5.40 2010-07-12 r3124 [x86_64-unknown-linux-gnu] (local build)
...

root@toy:/ssd/data# du -skh pg_xlog/
4.2G    pg_xlog/

root@toy:/ssd/data# smartctl -a /dev/sdg1 | grep LBAs
241 Total_LBAs_Written      0x0032   100   100   000    Old_age   
Always       -       18128
242 Total_LBAs_Read         0x0032   100   100   000    Old_age   
Always       -       10375

root@toy:/ssd/data# cat pg_xlog/* > /dev/null

root@toy:/ssd/data# smartctl -a /dev/sdg1 | grep LBAs
241 Total_LBAs_Written      0x0032   100   100   000    Old_age   
Always       -       18128
242 Total_LBAs_Read         0x0032   100   100   000    Old_age   
Always       -       10508

That's an increase of 133 after reading 4.2GB of data, which means makes 
each LBA turn out to be 32MB in size.  Let's try to confirm that by 
doing a write:

root@toy:/ssd/gsmith# smartctl -a /dev/sdg1 | grep LBAs
241 Total_LBAs_Written      0x0032   100   100   000    Old_age   
Always       -       18159
242 Total_LBAs_Read         0x0032   100   100   000    Old_age   
Always       -       10508
root@toy:/ssd/gsmith# dd if=/dev/zero of=test_file.0 bs=32M count=25 && sync
25+0 records in
25+0 records out
838860800 bytes (839 MB) copied, 5.95257 s, 141 MB/s
root@toy:/ssd/gsmith# smartctl -a /dev/sdg1 | grep LBAs
241 Total_LBAs_Written      0x0032   100   100   000    Old_age   
Always       -       18184
242 Total_LBAs_Read         0x0032   100   100   000    Old_age   
Always       -       10508

18184 - 18159 = 25; exactly the count I used in 32MB blocks.

-- 
Greg Smith   2ndQuadrant US    greg@2ndQuadrant.com   Baltimore, MD





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