[OpenIndiana-discuss] Good SLOG devices?
matthew.lagoe at subrigo.net
Thu Mar 3 09:59:05 UTC 2011
To be frank if you get 2 years of use out of a drive, while that's not a
huge amount of time, its still quite good. Also in 2 years you will have
SSD's that you can replace it with for next to nothing that will be even
In regards to your 2nd question it is my understanding that the SLOG is
written to so that the OS can return a "write complete". ZFS only does a
write cycle once every 5 seconds, as such if you happened to lose power in
that 5 seconds you would lose all that data. the SLOG basically buffers the
writes to the SSD so in the event of power loss ect, the data is not lost
and is read from and written to the zpool on startup.
My numbers may not be exact above but the general idea is that rather then
writing to the slower drives, it buffers the writes to the fast SSD then
slowly writes to the zpool, making it so that especially small writes are
still fast, as the OS returns a "write complete" before its written to the
slow drives, while at the same time ensuring your data integrity with the
From: "wessels" <wessels147 at gmail.com>
Sent: Wednesday, March 02, 2011 5:54 PM
To: "Discussion list for OpenIndiana"
<openindiana-discuss at openindiana.org>
Subject: Re: [OpenIndiana-discuss] Good SLOG devices?
I think that's the correct way to model the life expectancy.
Also factor in the economical life of IT hardware, 2 to 3 years in
And don't forget how quickly technology becomes obsolete these days.
I really wouldn't recommend MLC memory for a SLOG.
On Thu, Mar 3, 2011 at 12:58 AM, Ivar Janmaat <ijanmaat at xs4all.nl> wrote:
> Hello Wessel,
> I have some limited time right now so I thought why can't I calculate
> wear with what I know.
> Kown values:
> Write IOPS per Windows VM = 5
> Number of VMs = 100
> Write blocks between 4 and 32 KB (lets assume the worst: 32)
> MLC: 10.000 write cycles
> SLC: 100.000 write cycles
> Can I do the following math?
> Writen: 5x100x32 = 16 MB/s
> Total amount of data which can be written to a MLC: 10.000 x 100 GB
> (capacity) = 1.000.000 GB
> Total amount of data which can be written to a SLC: 100.000 x 100 GB
> (capacity) = 10.000.000 GB
> Lifetime MLC: 1.000.000.000 MB / 16 MB/s = 723 Days = 2 years
> Lifetime SLC: 10.000.000.000 MB / 16 MB/s = 7230 Days = 20 years
> So even without testing I think the MLCs are a no go.
> I need to test if the real iops are not 10 times higher than I expect
> otherwise the SLC is also a no go.
> One other question:
> You said that the ZIL is only read on reboot.
> But how does the ZIL work then? How is the data transfered /
> with the real harddisk? Or is that not done at all?
> Kind regards,
> OpenIndiana-discuss mailing list
> OpenIndiana-discuss at openindiana.org
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