Re: SSD filesystem aligned to DBMS - Mailing list pgsql-general

On Tue, Jan 16, 2018 at 08:02 Scott Marlowe <scott.marlowe@gmail.com> wrote:

On Tue, Jan 16, 2018 at 7:47 AM, Neto pr <netoprbr9@gmail.com> wrote:
> Hi all
>
> Sorry, but I'm not sure that this doubt is appropriate for this list, but I
> do need to prepare the file system of an SSD disk in a way that pointed me
> to, which would be a way optimized SSD
>  to work. I have a disk: SSD: Samsung 500 GB SATA III 6Gb/s - Model: 850 Evo
> http://www.samsung.com/semiconductor/minisite/ssd/product/consumer/850evo/
>
> One person on the list me said that should be partition aligned to 3072 not
> default 2048, to start on erase block bounduary. And fs block should be 8kb.
>
> Can you give me a hint of what program I could do this. I have already used
> fdisk but I do not know how to do this in Fdisk. I used Linux Debian
> 8(Jessie) 64b with Ext4 File system.

fdisk is pretty old and can't handle larger disks. You can get a fair
bit of control over the process with parted, but it takes some getting
used to. As far as I know, linux's ext4 has a maximum block size of
4k. I can't imagine alignment matters to SSDs and I would take any
advice as such with a large grain of salt and then if I had questions
about performance I'd test it to see. I'm willing to bet a couple
bucks it makes ZERO difference.

Alignment definitely makes a difference for writes. It can also make a difference for random reads as well since the underlying read may not line up to the hardware add in a read ahead (at drive or OS Level) and you’re reading far more data in the drive than the OS asks for.

Stupidly a lot of this isn’t published by a lot of SSD manufacturers, but through benchmarks it shows up.

Another potential difference here with SAS vs SATA is the maximum queue depth supported by the protocol and drive. 

SSD drives also do internal housekeeping tasks for wear leveling on writing.

I’ve seen SSD drives benchmark with 80-90MB sequential read or write, change the alignment, and you’ll get 400+ on the same drive with sequential reads (changing nothing else)

A specific example https://www.servethehome.com/ssd-alignment-quickly-benchmark-ssd/

>
> If you prefer, just reply to me, since the subject would not be about
> postgresql itself. netoprbr9@gmail.com

No this affects everybody who uses SSDs so let's keep it on list if we can.

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