What is BBU / Write Back??? (RAID)

gogoav

小弟未玩過raid
但呢排想買張hardware raid咭做公司server

見到raid咭有時會跟BBU
又話可以"玩write back"

係咩意思呢？
請指教

sony_eng

BBU = Battery Backup Unit

Info from Wiki :

Write cache reliability
The disk system can acknowledge the write operation as soon as the data is in the cache, not waiting for the data to be physically written. This typically occurs in old, non-journaled systems such as FAT32, or if the Linux/Unix "writeback" option is chosen without any protections like the "soft updates" option (to promote I/O speed whilst trading-away data reliability). A power outage or system hang such as a BSOD can mean a significant loss of any data queued in such cache.
Often a battery is protecting the write cache, mostly solving the problem. If a write fails because of power failure, the controller may complete the pending writes as soon as restarted. This solution still has potential failure cases: the battery may have worn out, the power may be off for too long, the disks could be moved to another controller, the controller itself could fail. Some disk systems provide the capability of testing the battery periodically, however this leaves the system without a fully charged battery for several hours.
An additional concern about write cache reliability exists, specifically regarding devices equipped with a write-back cache—a caching system which reports the data as written as soon as it is written to cache, as opposed to the non-volatile medium.[18] The safer cache technique is write-through, which reports transactions as written when they are written to the non-volatile medium.

pcm10241024

no BBU also use "write back"

inori

原帖由 gogoav 於 2009-4-26 04:11 PM 發表
小弟未玩過raid
但呢排想買張hardware raid咭做公司server

見到raid咭有時會跟BBU
又話可以"玩write back"

係咩意思呢？
請指教

簡單講write back比較快,缺點係一停/斷電(or hang機?),寫入中既資料就會冇哂
ibbu(電池)既作用係係冇電既時候作緊急後備電源,等將寫入中既資料寫哂入raid就斷電
現時好多raid咭就算冇ibbu都可以強制write back,
但如果有ups,ibbu既作用好似不大

[ 本帖最後由 inori 於 2009-4-27 10:06 編輯 ]

gogoav

有少少唔明

UPS係咪指類似APC 0個D
插左UPS就可以代替個BBU???

madox

原帖由 inori 於 2009-4-27 10:06 發表


簡單講write back比較快,缺點係一停/斷電(or hang機?),寫入中既資料就會冇哂
ibbu(電池)既作用係係冇電既時候作緊急後備電源,等將寫入中既資料寫哂入raid就斷電
現時好多raid咭就算冇ibbu都可以強制write back,
但如果有ups,ibbu既作用好似不大

如果係火牛壞, ibbu就好有用

Jason@YIU_SHUN

原帖由 gogoav 於 2009-4-27 20:59 發表
有少少唔明

UPS係咪指類似APC 0個D
插左UPS就可以代替個BBU???

不 , 我插了 ups 開 write back 還是有 warning message

and if ur mb out of order , ups still cannot protect your raid card