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  1. #1

    Adaptec 2405 - write cache enabled in RAID 10?

    The Adaptec 2405 gives heavy warnings about not enabling the write cache without battery backup on the RAID card. In a RAID 10 setup (4 drives), is this a risk? I see alot of people are running these in RAID 10, do they enable the write cache?
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  2. #2
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    The drawback is that without BBU in this arrangement, there is a good chance the array may have to rebuild after a hard power loss. The only risk would be 1. losing a disk during rebiuld (unlikely), or 2. the load of the rebuild process. As far as actual data loss there is little risk either way, and the BBU isn't going to guarantee a healthy array after power loss. Power loss is bad bad bad any way you look at it or try to prepare for it. The risk of filesystem corruption will always be there no matter what.
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  3. #3
    Actually, with a correctly functioning card, a battery on the card would prevent the need for a RAID rebuild after a power outage. If you experience otherwise, either your card or drives are not configured right or are non-compliant. Filesystem corruption also should not happen at all with a filesystem designed to ensure a consistent state at all times (via journal or BSD softupdates). Only caveat is that the filesystem designers or OS implementers will define what possible errors you must tolerate. For instance, ext4 is unsafe for certain application write patterns during a power outage, while ext3 in the same situation would be fine.

    Some Adaptec cards have an option to enable or disable the write cache on the disks. If so, it should be off. This setting is separate from enabling write caching on the card itself, which is perfectly safe to activate if you have an on-board battery.

    If you enable write caching on the card without a battery, you risk not just RAID rebuilds, but filesystem corruption. Your RAID configuration does not change this.

  4. #4
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    It's a sad day when this is discussed.

    You do risk major filesystem corruption and a few other things by enabling it but by disabling it you take a huge huge huge performance hit.
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  5. #5
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    Adaptec 2405 does not have an available BBU. However if your box is on a sexy UPS you'll probably be fine, even better with redundant power supplies on that as well. Still a card + BBU is the preferred method.

  6. #6
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    You can cause serious file system corruption if you hard reboot (Eg, via an APC), under high system load and do not have a BBU installed.
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  7. #7
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    Quote Originally Posted by Jay Suds View Post
    You can cause serious file system corruption if you hard reboot (Eg, via an APC), under high system load and do not have a BBU installed.
    Quote Originally Posted by MikeTrike View Post
    Still a card + BBU is the preferred method.
    Also the 2405 is not capable of a BBU as noted.

  8. #8
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    The RAID level is almost irrelevant.

    When the operating system flushes data from its own cache to "disk", the RAID controller may store the data in its own write cache for a short time before actually writing the data to disk.

    If the power fails before the RAID controller has synced its cache to disk, then you essentially lose data that the operating system thinks has been durably written to disk.

    Having a BBU allows the RAID controller to retain this cached data even during a power failure and subsequently write the data to disk after power is restored to the array.

    Conclusion: You may completely lose data if you use write caching without a BBU.

  9. #9
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    I don't get what the array rebuilding has to do with a BBU. As others have said not having a BBU and enabling write caching is a good way to get corruption when the machine is power-cycled OR rebooted(crashes, swap thrashes itself to death, or just loses power).

    I have seen many a file-system die due to this. The sad part is that these machines did have BBU's but unfortunately when they crashed it was because the controller completely locked up (LSI SAS controller) and during the bootup it complains that there is a problem with the BBU and disk cache was lost.

    Of course the BBU is actually fine but the when the controller completely craps out it takes its disk cache with it. Best case is waiting through a long file-system check. Medium case is having to move the data to a new file-system after the file-system going to read-only again after a fsck. Worst case file-system is completely fux0rd and you cant get any useful data off of it (or all directory structure is gone and everything is in lost + found with no filenames).


    So long story short. Don't run use write caching if you have data you care about and don't have a backup handy.

  10. #10
    Quote Originally Posted by houkouonchi View Post
    So long story short. Don't run use write caching if you have data you care about and don't have a backup handy.
    5% of the risk comes from write caching. 95% of the risk comes from ignoring write cache flushes from the OS. When you use a RAID card, and enable the write cache on card, you also get the flush ignore automatically.

    You can also get the same thing to happen (and the same risk) by checking 'Enable Advanced Performance' on the disk in certain versions of Windows. Microsoft doesn't give enough warning about the risk, although they fixed that in Windows 7.

  11. #11
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    Well enabling that on windows basically is exactly the same thing (write caching). So its just like a non-raid controller version of the same issue.

    Were you just making a warning to windows users?

  12. #12
    Quote Originally Posted by houkouonchi View Post
    Well enabling that on windows basically is exactly the same thing (write caching). So its just like a non-raid controller version of the same issue.

    Were you just making a warning to windows users?
    Warning applies to everybody, but Windows users have a uniquely easy way of falling into the massive corruption trap without even owning a RAID card. Because 'enable advanced performance' isn't labeled correctly, the risk is unrecognized.

    Windows lets you enable write caching without ignoring write cache flushes. A hardware RAID card is not the OS, so it can not semantically do the same. That's why a RAID card should not do write caching at all unless it has a battery.

    And for those not paying attention, 'Enable Advanced Performance' == ignore write cache flushes == big danger

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