I investigate bit about SSD and how it works and need to be aligned .
And I conclude that in the ideal world we need a general --ebs=xxx switch in various linux tools to ensure alignment. Or make calculation by had..
On the market there are SSD disks with page size 4 or 8 kb. But there is for ssd disk typical property - the EBS - Erase Block Size. If disk operate and write to single sector, whole Erase block must be read by driver electronic, modified and write back to the drive.
On the market there are devices with multiple EBS sizes . 128, 256, 512 1024 1534 2048 kib etc
In my case Samsung 850evo there are 8k pages and 1536 Erase Block
So first problem with alegment - partition should start on the Erase block bounduary . So --ebs switch in partition tools for propper aignment would be practical. Or calculate by hand. In my sase 1536 = 3072 512b sectors.
Things get complicate if You use mdadm raid. Because Raid superblock is located on the begining of the raid device and does not fill whole rerase block, it is practical to set in creation of raid --offset to real filesystem start at next erase block from the begining of raid device so underlying filesystem would be aligned as well. so --ebs=xxx on mdadm would be practice
And now ext4 so blocksize 4096 . because page size of ssd is 8kb , setting stride´wit is a smallest unit on with filesystem operate in one disk to 2 to fill ssd pagesize is practical. And stripe size set as ebs/pagesize or as whole ebs . and may be it would be useful to use ext4 --offset to edb as well.
this should align partition, raid and filesystem. fix me if I am wrong.
And now it is turn for database storage engine. I think try to write on erase block size bounduary and erase block size amount of data may have some benefits not with the speed but in lower wear-out of the entire ssd disk..
---------- Původní zpráva ----------
Od: Marcin Mańk <marcin.mank@xxxxxxxxx>
Komu: PostgreSQL <pgsql-general@xxxxxxxxxxxxxx>
Datum: 24. 11. 2015 20:07:30
Předmět: full_page_writes on SSD?
I saw this: http://blog.pgaddict.com/posts/postgresql-on-ssd-4kb-or-8kB-pagesIt made me wonder: if SSDs have 4kB/8kB sectors, and we'd make the Postgres page size equal to the SSD page size, do we still need full_page_writes?RegardsMarcin Mańk
