On Mon, May 1, 2017 at 5:56 PM, Sijie Guo <guosi...@gmail.com> wrote:
> I don't think this is an inconsistent issue. The in memory update is
> updating lac not current entry. Even the entry is added into memory but
> this entry will not be readable after lac is advanced, lac is advanced only
> after the next entry is added which happened after current entry is acked.
>
That is not true. You are talking about piggy-backed LAC only. But with
Explicit LAC
you don't need next entry to move LAC on bookie.
> So adding the entry to memory doesn't expose any consistency issue.
>
> On May 1, 2017 5:44 PM, "Venkateswara Rao Jujjuri" <jujj...@gmail.com>
> wrote:
>
> On Mon, May 1, 2017 at 2:31 PM, Yiming Zang <yz...@twitter.com.invalid>
> wrote:
>
> > Hi Andrey,
> >
> > That's a good point, and you're actually correct that if write to
> memTable
> > got throttled somehow, the addEntry request latency will be affected a
> lot.
> > This actually happens a few times in production cluster. Normally, the
> idea
> > of using Journal is to write data to the write-ahead log and then persist
> > the actual data to disks or add to memTable. However, my understanding of
> > why we choose to write entry to ledgerStorage first is to improve the
> > tailing-read performance.
> >
> > In SortedLedgerStorage.java, we first add entry to memTable and then we
> > update lastAddConfirmed, which means if there's a long poll read request
> or
> > readLastAddConfirmed request, it will immediately get satisfied for the
> > latest entry before we actually log the entry into Journal. So
> tailing-read
> > doesn't actually need to wait for any disk operation in Bookkeeper
> > including Journal operation.
> >
> > public long addEntry(ByteBuffer entry) throws IOException {
> > long ledgerId = entry.getLong();
> > long entryId = entry.getLong();
> > long lac = entry.getLong();
> > entry.rewind();
> > memTable.addEntry(ledgerId, entryId, entry, this);
> > ledgerCache.updateLastAddConfirmed(ledgerId, lac);
> > return entryId;
> > }
> >
> > But thinking about here, I'm wondering if it's actually safe to update
> the
> > LAC before we write the entry to Journal. What if we tell the client the
> > LAC has been updated but we actually failed to write the entry to Journal
> > and Bookie crashed at that time? Would this bring any inconsistency
> issue?
> >
>
> Good point. This is indeed an inconsistency issue. BK guarantees "if you
> read once you can read it all the time".
> If it is really done for LAC it is not really good idea. Unless I am
> missing something, this must be changed ASAP.
>
> Thanks,
> JV
>
>
> >
> > On Mon, May 1, 2017 at 2:13 PM, Andrey Yegorov <andrey.yego...@gmail.com
> >
> > wrote:
> >
> > > Hi,
> > >
> > > Looking at the code in Bookie.java I noticed that write to journal
> (which
> > > is supposed to be a write-ahead log as I understand) happened after
> write
> > > to ledger storage.
> > > This looks counter-intuitive, can someone explain why is it done in
> this
> > > order?
> > >
> > > My primary concern is that ledger storage write can be delayed (i.e.
> > > EntryMemTable's addEntry can do throttleWriters() in some cases) thus
> > > dragging overall client's view of add latency up even though it is
> > possible
> > > that journal's write (i.e. in case of dedicated journal disk) will
> > complete
> > > faster.
> > >
> > > private void addEntryInternal(LedgerDescriptor handle, ByteBuffer
> > > entry, WriteCallback cb, Object ctx)
> > >
> > > throws IOException, BookieException {
> > >
> > > long ledgerId = handle.getLedgerId();
> > >
> > > entry.rewind();
> > >
> > > *// ledgerStorage.addEntry() is happening here*
> > >
> > > long entryId = handle.addEntry(entry);
> > >
> > >
> > > entry.rewind();
> > >
> > > writeBytes.add(entry.remaining());
> > >
> > >
> > > LOG.trace("Adding {}@{}", entryId, ledgerId);
> > >
> > > *// journal add entry is happening here*
> > >
> > > *// callback/response to client is sent after journal add is done.*
> > >
> > > journal.logAddEntry(entry, cb, ctx);
> > >
> > > }
> > >
> > >
> > >
> > > ----------
> > > Andrey Yegorov
> > >
> >
>
>
>
> --
> Jvrao
> ---
> First they ignore you, then they laugh at you, then they fight you, then
> you win. - Mahatma Gandhi
>
--
Jvrao
---
First they ignore you, then they laugh at you, then they fight you, then
you win. - Mahatma Gandhi
