Why IO tests are overrated.
OK, this is going to be long but I feel it is an issue that has to be raised. We see more and more ppl posting dd tests, iopings and things like those and they all think that being unable to write a 1 GB continuous random file at speeds above 100 MB/s or ioping spikes once in a while means the storage is bad, host oversold and whatnot. I agree I am looking more for bw and dont care much about storage speed as long as it is in good supply, but asking huge speeds from our hosts at LEB prices we are actually lowering the storage capacity indirectly. They put expensive SSDs or SAS2 arrays, as such space is not great (even tho the ssd cache technique seems promising).
Question is, who needs that much speed ? Will my blog load slower if the "DD" speed is below 100 ? Or below 20 for that matter ? Do I really need to write fast for my site to be snappy ? First, write speed for RAID is way slower than read, usually, and for serving pages or anything from the web, means you need read and not write speed. Writing big (1GB+) files is not likely to happen in a production environment, will only matter in backups of some sorts, and, even then, the speed will be limited by other factors, such as encryption or compressing, making it highly unlikely you will be able to generate the file at speeds close to 100 MB/s, not to mention download from somewhere on shared ports of 1 gbps or below.
Agreed, these tests give an IDEA about the quality of the storage and the level of "crowding" on the node, but only an idea and not a verdict. A regular SATA 3 array will cost 5 times less per GB and if I have to choose between 20 GB sas2 over 100 GB SATA 3 with a 1.5-2 times lower "DD" speed, I will always get the higher storage. On the other hand, if i need fast DB or a game server, I choose SSD and this is it.
Next time you are bashing hosts for "slow" "DD", think if you need a faster one and if not the capacity would be a good trade-off over speed. Read operations are 5 times more frequent than write operations, and that is where the performance should be monitored as well as in ioping for responsiveness, but if you see a spike out of 10, it does not matter, in a shared server, it ALWAYS happens that someone is doing some IO intensive operation, if you need constant iopings, go dedicated.
In conclusion: I have high hope regarding the SSD cache thing, that can speed up cheap SATA 3 arrays, a good quality drives will mean cheap and solid storage, but, int he meantime, please dont force providers to buy SAS2 drives and reduce storage just because you want to win in the dd race, we are supposed to give a trend to the market, to show the way for thousands of ppl reading here to make the difficult choice of providers. Even so, hosts should consider have more kinds of offers, some with high speed storage (SSD or SSD cached) and whose with plenty of space, (SATA 3 with or without SSD cache). I will always choose the higher space offer, over the one 2 times as fast. M
I work for Uncle!