Re: Tuning guidelines for server with 256GB of RAM and SSDs?

<p dir="ltr">Hi, <p dir="ltr">We had a similar situation and the best performance was with 64MB background_bytes and
512MB dirty_bytes.<p dir="ltr">Tigran.<br /><div class="gmail_quote">On Jul 5, 2016 16:51, Kaixi Luo
<kaixiluo@gmail.com>wrote:<br type="attribution" /><blockquote class="quote" style="margin:0 0 0
.8ex;border-left:1px#ccc solid;padding-left:1ex"><div dir="ltr">Hello,<div><br /></div><div>I've been reading Mr. Greg
Smith's"Postgres 9.0 - High Performance" book and I have some questions regarding the guidelines I found in the book,
becauseI suspect some of them can't be followed blindly to the letter on a server with lots of RAM and
SSDs.</div><div><br/></div><div>Here are my server specs:</div><div><br /></div><div><div>Intel Xeon E5-1650 v3 <span
style="line-height:1.5">Hexa-CoreHaswell </span></div></div><div>256GB DDR4 ECC RAM</div><div>Battery backed hardware
RAIDwith 512MB of WriteBack cache (LSI MegaRAID SAS 9260-4i)</div><div>RAID1 - 2x480GB Samsung SSD with power loss
protection(will be used to store the PostgreSQL database)</div><div>RAID1 - 2x240GB Crucial SSD with power loss
protection.(will be used to store PostgreSQL transactions logs)</div><div><br /></div><div>First of all, the book
suggeststhat I should enable the WriteBack cache of the HWRAID and disable the disk cache to increase performance and
ensuredata safety. Is it still advisable to do this on SSDs, specifically the step of disabling the disk cache?
Wouldn'tthat increase the wear rate of the SSD?</div><div><br /></div><div>Secondly, the book suggests that we increase
thedevice readahead from 256 to 4096. As far as I understand, this was done in order to reduce the number of seeks on a
rotatinghard drive, so again, is this still applicable to SSDs?</div><div><br /></div><div>The other tunable I've been
lookinginto is vm.dirty_ratio and vm.dirty_background_ratio. I reckon that the book's recommendation to lower
vm.dirty_background_ratioto 5 and vm.dirty_ratio to 10 is not enough for a server with such big amount of RAM. How much
lowershould I set these values, given that my RAID's WriteBack cache size is 512MB?</div><div><br /></div><div>Thank
youvery much.</div><div><br /></div><div>Kaixi Luo</div></div></blockquote></div>