Disk Write Cache [was: Re: RAID1 σε Linux για αρχεία που αλλάζουν σπάνια]

[V13]

On Thursday 16 July 2009, Christos Ricudis wrote:
> Giannis Papadopoulos wrote:
> > 
> > http://www.linuxquestions.org/questions/linux-software-2/reiserfs-slight-
> >occasional-corruption-on-software-raid-5-arrays-272848/
>
> "E nai mwre afou einai slight kai occassional den peirazei".

FWIW:

Opoiadhpote xrhsh skliroy diskoy se PC me energopoihmenh th write-cache, me h 
xoris RAID, me h xoris reiserfs, me h xoris to X/Y/Z, prokalei slight and 
occassional corruption

Otan enas skliros diskos exei (p.x.) 16MB cache shmainei pos mporei kai 
krataei eos 16MB ta opoia den exei grapsei akomi sthn epifaneia. An yparksei 
diakopi reymatos tote xanontai eos 16MB dedomena. Ekei ofeiletai kai to power-
off ton diskon kata to shutdown. Ginetai gia na einai sigoyro oti 
apothikeytikan ta dedomena stoys diskoys giati to aplo 'sync' den arkei.

Th shmeron hmeran oi diskoi kanoyn kai alles patentes opos reordering sta 
dedomena ths cache gia na ta grapsoyn grigorotera. Ayto exei san apotelesma na 
exeis pei na apothikeytoyn ta 1,2,3,4,5 kai ayta na graftoyn me th seira 
1,5,3,4,2. An tora o diskos klisei eno exei pragmata sth cache toy gia 
grapsimo, den mporeis na eisai sigoyros gia to ti exei graftei. H symperifora 
ayth axristevei se poly megalo bathmo thn idea toy transaction log mias kai 
ekei apaiteitai na enimeronetai to log prin ginei kapoia allagei sto disko. 
Edo ofeiletai kai to corruption.

Ta parapana exoyn pio emfaneis epiptoseis sth periptosi toy RAID. Sth kaliteri 
periptosi, ena RAID-5 me N diskoys pollaplasiazei th pithanotita na paei kati 
straba x N. Ena apo ta pragmata poy kanoyn oi hardware RAID controllers einai 
na kleinoyn th write-cache ton diskon kai merikes fores dinoyn kai th 
dynatotha epilogis (p.x. Adaptec). An tora theleis na to apofygeis en-meri 
ayto, bazeis controller me cache+mpataria.

Profanos kai an pistevei kaneis pos xoris RAID  kai me energopoihmenh th 
write-cache einai asfalhs einai gelasmenos. Isos apla na min paratirei ayta 
poy symbainoyn.

Kleinontas gia to RAID: To RAID se profylasei apo disko o opoios xtypaei. Den 
se profylasei oyte apo lathi sthn epifaneia toy diskoy (p.x. dedomena ta opoia 
den diabazontai sosta alla diabazontai) oyte apo diakopes reymatos.

Ta apotelesmata toy apotomoy power-off mporoyn poly eykola na dokimastoyn an 
baleis to filesystem ths epilogis soy se enan eksoteriko USB disko kai eno 
exeis I/O (h apla den exei ginei umount) ton aposyndeseis (epanalabate X fores 
opoy X>=10). Eksomeioneis diladi th katastash xoris reboot.

[1] http://sr5tech.com/write_back_cache_experiments.htm
[2] http://lwn.net/Articles/283161/
[3] http://www.google.com/search?q=raid+write+cache

<<V13>>