[OpenIndiana-discuss] RAM based devices as ZIL

[Steve Gonczi]

For a fast (high ingest rate) system, 4G may not be enough. 

If your Ram device space is not sufficient to hold all the in-flight Zill blocks, 
it will fail over, and the Zil will just redirect to your main data pool. 

This is hard to notice, unless you have an idea of how much 
data should be flowing to your pool, as you monitor it with 
zpool iostat. Then you may notice the extra data being written to your 
data pool. 

The calculation of how much Zil space you need is not straight forward, 
because blocks in general are freed in a delayed manner. 

In other words, it is possible that the some Zil blocks are no longer needed 
because the transactions they represent already committed, but the blocks 
have not made it back to "available" status because of the conservative 
nature of the freed block recycling algo. 

Rule of thumb, 3 to 5 txg-s worth of ingest, depending on who you ask. 

Dedup and compression makes Slog sizing harder, because the Zil is neither 
compressed nor deduped. I would say if you dedup and / or compress, 
all bets are off. 



/sG/ 

----- Original Message -----
Hello, 

Does anyone have any real world experience using a RAM based device like the 
DDRdrive X1 as a ZIL on 151a7? At 4GB they seem to be a little small but with 
some txg commit interval tweaking it looks like it may work. The entire 4GB is 
saved to NAND in case of a power failure so it seems like a pretty safe solution 
(entire system is on UPS and generator anyway). 

Thanks, 

Wim 
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