Cache memory

Posted on 2012-09-07
Last Modified: 2012-10-25
Please explain the different of item below and how does it work,why we need those item in server.

1.Flash Backed Write Cache
2.Flash Backed Cache
Question by:warriorsTeah
    LVL 55

    Accepted Solution

    1 and 2 are the same, FBWC.
    A supercapacitor or small battery is connected to the controller's cache and there is a NAND flash chip on the cache too. When power is lost the contents of the DRAM cache is written to the flash chip before the supercap goes flat. Once there it's indefinitely saved to be flushed to disk on next power on.

    3 Battery Backed Write Cache.
    A relatively large battery is connnected to the cache to keep it powered during unexpected outages and retain the write cache contents to be written to disk on next power on. Since the battery is powering the DRAM and associated refresh circuit it eventually goes flat and the contents are lost if power is not restored soon enough. 72 hours is a common lifetime for a full cache battery before it goes flat.

    Author Comment

    For the FBWC, how long data can keep and we still need to buy other component to work it together ?
    LVL 55

    Expert Comment

    Already said data does not expire in FBWC, it can last years. When the power comes back on it is flushed to disk. if the controller has failed you would need a similar one to replace it with and you can transfer the FBWC to that and it will flush to disk as normal assuming the same disks are present.

    Author Comment

    In what situation we need BBWC and izit the FBWC is the best choice must include in  my server
    LVL 55

    Expert Comment

    FBWC is always better than BBWC, it just wasn't available a few years ago so isn't available for all makes/models of controller.

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