Re: SSDs with Postgresql?

On 22/04/11 01:33, Florian Weimer wrote:
> * Greg Smith:
>
>> The fact that every row update can temporarily use more than 8K means
>> that actual write throughput on the WAL can be shockingly large.  The
>> smallest customer I work with regularly has a 50GB database, yet they
>> write 20GB of WAL every day.  You can imagine how much WAL is
>> generated daily on systems with terabyte databases.
>
> Interesting.  Is there an easy way to monitor WAL traffic in away?  It
> does not have to be finegrained, but it might be helpful to know if
> we're doing 10 GB, 100 GB or 1 TB of WAL traffic on a particular
> database, should the question of SSDs ever come up.

One thought I had on monitoring write usage..

If you're on Linux with the ext4 filesystem, then it keeps track of some
statistics for you.
Check out /sys/fs/ext4/$DEV/lifetime_write_kbytes
(where $DEV is the device the fs is mounted on, eg. sda1, or dm-0, or
whatnot - see /dev/mapper to get mappings from LVMs to dm-numbers)

If you log that value every day, you could get an idea of your daily
write load.

-Toby