From: Andreas Dilger Date: Thu, 12 Sep 2019 09:00:13 +0000 (-0600) Subject: LUDOC-11 config: discourage on-board device cache X-Git-Tag: 2.13.0~5 X-Git-Url: https://git.whamcloud.com/?a=commitdiff_plain;h=2865c6061d1c7e1449fc563bc719630ba5e540af;p=doc%2Fmanual.git LUDOC-11 config: discourage on-board device cache Recommend against any cache onboard internal HBA devices. Signed-off-by: Andreas Dilger Change-Id: Ia34e2fffb9008b360343a27d4cf599ec6f3fe0ea Reviewed-on: https://review.whamcloud.com/36168 Tested-by: jenkins Reviewed-by: Joseph Gmitter --- diff --git a/ConfiguringStorage.xml b/ConfiguringStorage.xml index 00e7bee..9c2921e 100644 --- a/ConfiguringStorage.xml +++ b/ConfiguringStorage.xml @@ -62,9 +62,24 @@
<indexterm><primary>storage</primary><secondary>performance tradeoffs</secondary></indexterm>Performance Tradeoffs - A writeback cache can dramatically increase write performance on many types of RAID arrays if the writes are not done at full stripe width. Unfortunately, unless the RAID array has battery-backed cache (a feature only found in some higher-priced hardware RAID arrays), interrupting the power to the array may result in out-of-sequence writes or corruption of RAID parity and future data loss. - If writeback cache is enabled, a file system check is required after the array loses power. Data may also be lost because of this. - Therefore, we recommend against the use of writeback cache when data integrity is critical. You should carefully consider whether the benefits of using writeback cache outweigh the risks. + A writeback cache in a RAID storage controller can dramatically + increase write performance on many types of RAID arrays if the writes + are not done at full stripe width. Unfortunately, unless the RAID array + has battery-backed cache (a feature only found in some higher-priced + hardware RAID arrays), interrupting the power to the array may result in + out-of-sequence or lost writes, and corruption of RAID parity and/or + filesystem metadata, resulting in data loss. + + Having a read or writeback cache onboard a PCI adapter card installed + in an MDS or OSS is NOT SAFE in a high-availability + (HA) failover configuration, as this will result in inconsistencies between + nodes and immediate or eventual filesystem corruption. Such devices should + not be used, or should have the onboard cache disabled. + If writeback cache is enabled, a file system check is required + after the array loses power. Data may also be lost because of this. + Therefore, we recommend against the use of writeback cache when + data integrity is critical. You should carefully consider whether the + benefits of using writeback cache outweigh the risks.