In all other cases controller and disks in one circuit an UPS would be superior?
Caches at various levels are involved in accessing data. They will be briefly described here.
When data is written, it will first be stored in this cache. The content of this cache is periodically transferred as well as when system calls like sync or fsync are called to the underlying memory system.
This underlying system may be a RAID controller or the hard disk directly. Under Linux, the number of megabytes of working memory currently used for the page cache is indicated in the Cached column of the report produced by the free -m command.
If the power fails, the content of the page cache will be lost. A typical example for such a cache would currently consist ofor MB.
If the power were to fail, the content of this cache would be lost, unless the content has been protected by a battery backup unit BBU or battery backup module BBM. BBUs and BBMs have integrated batteries, which can generally power the content of the cache for up to 72 hours.
Understanding controller caching and Exchange performance. Andrew Higginbotham. Write Cache: % of controller cache should be used for writes; Read Cache: 0% of controller cache should be used for reads; Stripe Size: K or . TTY Logs (the RAID controller log) contains references to read, write and cache policies assigned to virtual disks. These policies can impact the performance of virtual disks, and if not used properly, can increase the risk of data loss in the event of a power failure. Write-Through (WT) – Write. RAID Controller and Hard Disk Cache Settings. From Thomas-Krenn-Wiki. Unchecked. Jump to: navigation, When using a cache with the RAID controller, the cache content should be protected by a BBU or BBM. The battery status should be checked periodically. (Disk Write Cache Mode set to Enabled) Max. performance, data loss possible 5.
If the server is re-started during that period, the data in the cache can be recovered. The battery status should be checked at periodic intervals, since capacity will reduce over the life of the battery.
When the battery becomes too weak generally after one to three yearsit should be replaced just like a notebook battery. If the battery status is not checked, there is the risk after several years that the battery will only be able to retain the content of the cache for very short period, which would risk data loss if the power failure were to continue for a longer period.
RAID controllers, which do not use a BBU to protect the cache but instead copy the content of the cache to flash memory in the event of a power failuredo not require special cache protection maintenance.
Hard Disk Cache Hard disks also have an integrated cache. Newer 3ware controllers protect the content of caches integrated into hard disks using a proprietary Write Journal Storsave Configuration Settings.
Otherwise, the content of these caches would normally be lost upon power failure. Risks from the Loss of Cache Content during a Power Failure If the RAID controller or the hard disk were to inform the operating system that data had been written however the data had actually merely been stored in the cachethe worst-case scenario would involve complete loss of data during a sudden power failure.
The only choice in such a case would be to restore from the current backup. Cache settings for secure operation see below can reduce these risks, however they will quite naturally reduce performance somewhat. Settings for Secure Operation The objective of secure operation is to avoid the loss of data in the caches for the RAID controllers and the hard disks during a power failure.
The following ground rules should be used for secure operation. The battery status should be checked periodically. The batteries should be replaced every two or three years, as a general rule.
In addition, the fact that a BBU or BBM can only protect the content of the cache for up to 72 hours must be taken into consideration. If a power failure might last several days, the battery could be fully discharged, resulting in the loss of the cache content. For that reason, these caches should be deactivated.
With newer RAID controllers, 3ware provides a proprietary Write Journal feature, which will protect the cache content, when the Storsave settings have been set to Protection or Balanced in the case of the Balanced setting, only if a BBU is used.
The precise functional manner of this feature has not been document. Example Tests With the help of a Perl scriptwe were able to test the possibility of data loss during a power failure under Linux.
Background information regarding this script can be found at:Buy Dell PowerEdge RAID Controller H Electronics - timberdesignmag.com FREE DELIVERY possible on eligible purchases.
Which RAID caching policy to use? Ask Question. up vote 2 down vote favorite. The RAID write cache policy should be set depending on the disk I/O load. You must make sure that you have a battery on the RAID controller and that the battery is functional.
If it is not aligned, then the RAID controller must do a read-modify-write (read the stripe in, modify with the new data, write the stripe out), which has high overhead and latency. The purpose of RAID cache in this case is to hide the latency of writing to disk and receive acknowledgment as soon as the data hits the cache.
read policy: It's my understanding that "read ahead", "no read ahead", and whatever form of "Adaptive read ahead" you have are all pertaining to whether the RAID controller should try to read and cache a data stripe as it's accessed.
This seems silly and unnecessary considering that RAID 1 has no data striping. It doesn't power the disks, it just keeps the data in the cache for (in this case) up to 72 hours until you bring the machine back on line.
When you power the machine back up it will write the contents of the cache back out to the disks. ZFS is a combined file system and logical volume manager designed by Sun timberdesignmag.com is scalable, and includes extensive protection against data corruption, support for high storage capacities, efficient data compression, integration of the concepts of filesystem and volume management, snapshots and copy-on-write clones, continuous integrity checking and automatic repair, RAID-Z, native.