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https://issues.apache.org/jira/browse/HBASE-14283?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14744844#comment-14744844
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Ben Lau commented on HBASE-14283:
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Hey guys, I started looking into updating the HFile serialization to support
reverse scans per previous comments. One thing that immediately struck me as
being a possible problem is that the header sizes appear to be hardcoded into
HConstants.java (HConstants.HFILEBLOCK_HEADER_SIZE), rather than being read
from the HFile block header or HFile metadata itself.
This seems to imply that if I add more fields to the header and then do a
rolling restart to update all region servers to have my code, any old region
server that hasn't updated yet and is processing the new HFiles will not
realize the header is bigger now and that there is stuff they need to skip /
ignore. This might necessitate a 2-step restart process with 2 rolling
restarts.
Restart 1 to update all RS to have the appropriate new reading code. Restart 2
will enable writes by setting an HBase config option (false by default) to
start writing the new HFiles. Am I missing something and this 2-step rolling
restart is not necessary for some reason? It seems unlikely people would find
this process palatable but is there a better alternative?
Alternatively I can turn this into a non-backwards compatible major version
update instead of a minor version update and require a full cluster restart but
that is kind of harsh in its own way. Opinions/thoughts?
> Reverse scan doesn’t work with HFile inline index/bloom blocks
> --------------------------------------------------------------
>
> Key: HBASE-14283
> URL: https://issues.apache.org/jira/browse/HBASE-14283
> Project: HBase
> Issue Type: Bug
> Reporter: Ben Lau
> Assignee: Ben Lau
> Attachments: HBASE-14283-v2.patch, HBASE-14283.patch,
> hfile-seek-before.patch
>
>
> Reverse scans do not work if an HFile contains inline bloom blocks or leaf
> level index blocks. The reason is because the seekBefore() call calculates
> the previous data block’s size by assuming data blocks are contiguous which
> is not the case in HFile V2 and beyond.
> Attached is a first cut patch (targeting
> bcef28eefaf192b0ad48c8011f98b8e944340da5 on trunk) which includes:
> (1) a unit test which exposes the bug and demonstrates failures for both
> inline bloom blocks and inline index blocks
> (2) a proposed fix for inline index blocks that does not require a new HFile
> version change, but is only performant for 1 and 2-level indexes and not 3+.
> 3+ requires an HFile format update for optimal performance.
> This patch does not fix the bloom filter blocks bug. But the fix should be
> similar to the case of inline index blocks. The reason I haven’t made the
> change yet is I want to confirm that you guys would be fine with me revising
> the HFile.Reader interface.
> Specifically, these 2 functions (getGeneralBloomFilterMetadata and
> getDeleteBloomFilterMetadata) need to return the BloomFilter. Right now the
> HFileReader class doesn’t have a reference to the bloom filters (and hence
> their indices) and only constructs the IO streams and hence has no way to
> know where the bloom blocks are in the HFile. It seems that the HFile.Reader
> bloom method comments state that they “know nothing about how that metadata
> is structured” but I do not know if that is a requirement of the abstraction
> (why?) or just an incidental current property.
> We would like to do 3 things with community approval:
> (1) Update the HFile.Reader interface and implementation to contain and
> return BloomFilters directly rather than unstructured IO streams
> (2) Merge the fixes for index blocks and bloom blocks into open source
> (3) Create a new Jira ticket for open source HBase to add a ‘prevBlockSize’
> field in the block header in the next HFile version, so that seekBefore()
> calls can not only be correct but performant in all cases.
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