zfs l2arc warmup

[Dmitry Morozovsky]

On Fri, 28 Mar 2014, mikej wrote:

> > [snip most again]
> > 
> > > Around ZFSv14-ish, the ability to import a pool with a missing ZIL was
> > > added.
> > > 
> > > Remember the flow of data in ZFS:
> > >   async write request --> TXG --> disk
> > >   sync write request --> ZIL
> > >                \--> TXG --> disk
> > > 
> > > All sync writes are written to the pool as part of a normal async TXG
> > > after
> > > its written sync to the ZIL.  And the ZIL is only ever read during pool
> > > import.
> > 
> > On the other side, doesn't it put the risk on sync-dependent, like database,
> > systems?
> > 
> > I'm thinking not about losing the transaction, but possibly putting your
> > filesystem in the middle of (database PoV) transaction, hence render your DB
> > inconsistent?
> > 
> > Quick googling seems to be uncertain about it...
> 
> As I understand it..... (And I am always looking for an education)
> 
> Any files system that honors fsync and provided the DB uses fsync should be
> fine.
> 
> Any data loss then will only be determined by what transaction (log) 
> capabilities the DB has.

And? 

1. The DB issues "sync WAL" request, which is translated to fsync-like FS 
requests, there are (IIUC) should ne directed to ZIL.

2. ZIL is failing in the middle of the request, or, even more bad, after 
reporting that ZIL transaction is done, but before translating ZIL to the 
underlying media

3. inconsistend DB?

I'm in hope I'm wrong somewhere...

-- 
Sincerely,
D.Marck                                     [DM5020, MCK-RIPE, DM3-RIPN]
[ FreeBSD committer:                                 marck at FreeBSD.org ]
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*** Dmitry Morozovsky --- D.Marck --- Wild Woozle --- marck at rinet.ru ***
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