On 08/25/2012 11:53 AM, Jim Klimov wrote:
>> No they're not, here's l2arc_buf_hdr_t a per-buffer structure 
>> held for
>> buffers which were moved to l2arc:
>> typedef struct l2arc_buf_hdr {
>> l2arc_dev_t *b_dev;
>> uint64_t b_daddr;
>> } l2arc_buf_hdr_t;
>> That's about 16-bytes overhead per block, or 3.125% if the 
>> block's data is 512 bytes long.
>> The main overhead comes from an arc_buf_hdr_t, which is pretty fat,
>> around 180 bytes by a first degree approximation, so in all 
>> around 200
>> bytes per ARC + L2ARC entry. At 512 bytes per block, this is painfully
>> inefficient (around 39% overhead), however, at 4k average block size,
>> this drops to ~5% and at 64k average block size (which is entirely
>> possible on average untuned storage pools) this drops down to ~0.3%
>> overhead.
> So... unless I miscalculated before drinking a morning coffee, for a 512b 
> block
> quickly fetchable from SSD in both L2ARC and METAXEL cases, we have
> roughly these numbers?:
> 1) When it is in RAM, we consume 512+180 bytes (though some ZFS
> slides said that for 1 byte stored we spend 1 byte - i thought this meant zero
> overhead, though I couldn't imagine how... or 100% overhead, also quite
> unimaginable =) )
> 2L) When the block is on L2ARC SSD, we spend 180+16 bytes (though
> discussions about DDT on L2ARC at least, settled on 176 bytes of cache
> metainformation per entry moved off to L2ARC, with the DDT entry's size 
> being around 350 bytes, IIRC).
> 2M) When the block is expired from ARC and is only stored on the pool,
> including the SSD-based copy on a METAXEL, we spend zero RAM to
> reference this block from ARC - because we don't remember it anymore.
> And when needed, we can access it just as fast (right?) as from L2ARC
> on the same media type.
> Where am I wrong, because we seem to dispute over THIS point over 
> several emails, and I'm ready to accept that you've seen the code and 
> I'm the clueless one. So I want to learn, then ;)

The difference is that when you want to go fetch a block from a metaxel,
you still need some way to reference it. Either you use direct
references (i.e. ARC entries as above), or you use an indirect
mechanism, which means that for each read you will need to walk the
metaxel device, which is slow.

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