bcache should be the safe bet here if you ask me. none of them are discussed in this list, this is for md raid. databases do both but the relay on the filesystem confirming the write before they move on so you vm cache won't help here but the ssd will as the cache will confirm the write and make the database belive the data was in fact written to permanent storage. another thing you might be looking at aswell is recovery from powerfailure.... may I advice you to get a UPS for this setup. /Tommy 2013/9/18 Roberto Spadim <roberto@xxxxxxxxxxxxx>: > humm, i didn't search about dm cache and bcache devel status, what is more > production safe? bcache or dmcache? > dmcache should be discussed in linux-raid right? what about bcache? > > yes i'm thinking about ssd cache for small random writes, but i don't know > how a database do small random writes or it use only small sequencial > writes, i should consider this, right? i read some docs talking about only > random access being cached and sequencial are 'lost' and read from primary > disk (hdd in my case) > > > 2013/9/18 Tommy Apel <tommyapeldk@xxxxxxxxx> >> >> dm cache is AFAIK still Experimental, the size of the ssd won't matter >> too much in this case as you're only severing a 2TB backend and you'll >> be able to use sequential writes to the backing store at 150MB/s or >> so, once the writes are completed on the ssd they will be strained >> together and flushed to permanent storage at full sequential speed for >> the most time plus you'll have linux fs buffers as well so I'm not >> really too sure how much the ssd will actually contribute to the setup >> in total. The ssd caching will in most cases only benefit you if you >> have a lot of small random write IO. >> >> /Tommy >> >> >> 2013/9/18 Roberto Spadim <roberto@xxxxxxxxxxxxx>: >> > Sorry guys, this time i don't have a full knowledge about the >> > workload, but from what he told me, he want fast writes with hdd but i >> > could check if small ssd devices could help >> > After install linux with raid1 i will install apache mariadb and php >> > at this machine, in other words it's a database and web server load, >> > but i don't know what size of app and database will run yet >> > >> > Btw, ssd with bcache or dm cache could help hdd (this must be >> > enterprise level) writes, right? >> > Any idea what the best method to test what kernel drive could give >> > superior performace? I'm thinking about install the bcache, and after >> > make a backup and install dm cache and check what's better, any other >> > idea? >> > >> > Em 18/09/2013 11:00, "Drew" <drew.kay@xxxxxxxxx> escreveu: >> > >> >> > HDD will be a SATA 7200rpm with 2TB, SSD will be ~ 160GB or less >> >> > (must >> >> > be cheap and enterprise level) >> >> > >> >> > he wants a RAID1 level over HDD and a cache (bcache/or md cache layer >> >> > of kernel 3.9) over this md device (raid1) >> >> >> >> What is the client's workload? What is he trying to do with this >> >> machine? >> >> >> >> At first glance (without any information) the requirements don't make >> >> sense. Placing a SSD based cache in front of a pair of slow 7200RPM >> >> SATA's sounds like he's trying to get SSD performance at SATA prices. >> >> It could work but if he's hammering the disks enough to need SSD >> >> performance, those SATA's just won't keep up. >> >> >> >> >> >> -- >> >> Drew >> >> >> >> "Nothing in life is to be feared. It is only to be understood." >> >> --Marie Curie >> >> >> >> "This started out as a hobby and spun horribly out of control." >> >> -Unknown >> > >> > >> > -- >> > Roberto Spadim >> > SPAEmpresarial >> > -- >> > To unsubscribe from this list: send the line "unsubscribe linux-raid" in >> > the body of a message to majordomo@xxxxxxxxxxxxxxx >> > More majordomo info at http://vger.kernel.org/majordomo-info.html > > > > > -- > Roberto Spadim > SPAEmpresarial -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
