Does anyone have some time to review this 4 line change? Thanks,
Dave On 2022/12/07 19:06:00 "Dave Marion (Jira)" wrote: > Dave Marion created HDFS-16864: > ---------------------------------- > > Summary: HDFS advisory caching should drop cache behind block > when block closed > Key: HDFS-16864 > URL: https://issues.apache.org/jira/browse/HDFS-16864 > Project: Hadoop HDFS > Issue Type: Improvement > Components: hdfs > Affects Versions: 3.3.4 > Reporter: Dave Marion > > > One of the comments in HDFS-4817 describes the behavior in > BlockReceiver.manageWriterOsCache: > > "The general idea is that there isn't much point in calling > {{sync_file_pages}} twice on the same offsets, since the sync process has > presumably already begun. On the other hand, calling > {{fadvise(FADV_DONTNEED)}} again and again will tend to purge more and more > bytes from the cache. The reason is because dirty pages (those containing > un-written-out-data) cannot be purged using {{{}FADV_DONTNEED{}}}. And we > can't know exactly when the pages we wrote will be flushed to disk. But we do > know that calling {{FADV_DONTNEED}} on very recently written bytes is a > waste, since they will almost certainly not have been written out to disk. > That is why it purges between 0 and {{{}lastCacheManagementOffset - > CACHE_WINDOW_SIZE{}}}, rather than simply 0 to pos." > > Looking at the code, I'm wondering if at least the last 8MB (size of > CACHE_WINDOW_SIZE) of a block might be left without an associated > FADVISE_DONT_NEED call. We're having a > [discussion|https://the-asf.slack.com/archives/CERNB8NDC/p1669399302264189] > in #accumulo about the file caching feature and I found some interesting > [results|https://gist.github.com/dlmarion/1835f387b0fa8fb9dbf849a0c87b6d04] > in a test that we wrote. Specifically, that for a multi-block file using > setDropBehind with either hsync or CreateFlag.SYNC_BLOCK, parts of each block > remained in the cache instead of parts of the last block. > > I'm wondering if there is a reason not to call fadvise(FADV_DONTNEED) on the > entire block in close > [here|https://github.com/apache/hadoop/blob/trunk/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/BlockReceiver.java#L371] > when dropCacheBehindWrites is true. > > > > -- > This message was sent by Atlassian Jira > (v8.20.10#820010) > > --------------------------------------------------------------------- > To unsubscribe, e-mail: hdfs-dev-unsubscr...@hadoop.apache.org > For additional commands, e-mail: hdfs-dev-h...@hadoop.apache.org > > --------------------------------------------------------------------- To unsubscribe, e-mail: hdfs-dev-unsubscr...@hadoop.apache.org For additional commands, e-mail: hdfs-dev-h...@hadoop.apache.org
