Posts tagged hardware

write-back vs. write-through caching

posted on 2014-09-29 12:15:40

When putting hardware RAID controller to use, you usually have to choose between the option in the post title.

write-back

Once the hardware-RAID-controller (HWR) has the data cached, which the operating system sent, it tells the OS to go on. (The OS is told the disk write is finished.)

OS          --          HWR cache           --          HDD / SSD
1. send data to write
                        2. data is cached
                        3. OS is told to go on
4. OS goes on
                                                        5. data is written to disk,
                                                           when system load is low.

write-through

Only when the data has been written to disk, the OS is told so.

OS          --          HWR cache           --          HDD / SSD
1. send data to write
                        2. data is cached
                                                        3. data is written to disk
                        4. OS is told to go on
5. OS goes on

Test from the Proxmox wiki: (here)

Single SATA WD 400GB: 1360.17
3 x 15K rpm sas RAID5 with write-through: 159.03 (YES, only 159!)
same as above but with write-back enabled: 3133.45

TL;DR

Use write-back on controllers with a battery backup unit (BBU) or a flash memory, depending on which functionality is available.

Adaptec's lower series cannot be equipped with neither, the 5xxx series can have one of each (or even both?). The 6xxx series doesn't need a BBU.

Take note, that BBU's need attention, whereas flash-based solutions are maintenance-free.

With SSD's however, you can forget about all this and just use write-through, since it's fast enough.

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