CentOS - Feb 2015 - OT: AF 4k sector drives with 512 emulation

Still have good quality older sata hardware raid cards that require 512
bytes/sector. As far as I know HD manufacturers are not making native 512
bytes/sector drives any more.

Some have better 512e emulation than others. Looking for some advice on
which to avoid and which are recommended. Thanks. PS this is for a CentOS6
server.

On 2/27/2015 12:53 PM, Robert Arkiletian wrote:
> Still have good quality older sata hardware raid cards that require 512
> bytes/sector. As far as I know HD manufacturers are not making native 512
> bytes/sector drives any more.
>
> Some have better 512e emulation than others. Looking for some advice on
> which to avoid and which are recommended. Thanks. PS this is for a CentOS6
> server.
any of the 'enterprise' nearline storage or NAS drives should be fine.
I wouldn't use anything else in a RAID setup.

Seagate NS series, for instance, or WD Red or Re, etc.



-- 
john r pierce                                      37N 122W
somewhere on the middle of the left coast

On Fri, Feb 27, 2015 at 1:53 PM, Robert Arkiletian <robark at gmail.com>
wrote:
> Still have good quality older sata hardware raid cards that require 512
> bytes/sector. As far as I know HD manufacturers are not making native 512
> bytes/sector drives any more.
512n drives still exist, although they tend to be a bit smaller, 2TB or less.
http://www.hgst.com/tech/techlib.nsf/techdocs/FD3F376DC2ECCE68882579D40082C393/$file/US7K4000_ds.pdf


4Kn drives are appearing now also. I don't expect these drives to be
bootable except possibly by systems with UEFI firmware. It's also
possible hardware RAID will reject them unless explicitly supported.
http://www.hgst.com/tech/techlib.nsf/techdocs/29C9312E3B7D10CE88257D41000D8D16/$file/Ultrastar-7K6000-DS.pdf

> Some have better 512e emulation than others. Looking for some advice on
> which to avoid and which are recommended. Thanks. PS this is for a CentOS6
> server.
The emulation implementations don't come into play if the alignment is
correct from the start. The better implementations have significantly
less pathological behavior if alignment is wrong, but that's
anecdotal, I don't have any empirical data available. But I'd say in
any case you want it properly aligned.


-- 
Chris Murphy

Chris Murphy wrote:
<snip>
> The emulation implementations don't come into play if the alignment is
> correct from the start. The better implementations have significantly
> less pathological behavior if alignment is wrong, but that's
> anecdotal, I don't have any empirical data available. But I'd say
in
> any case you want it properly aligned.
You really, really want it properly aligned. We ran into that problem when
we started getting 3TB drives a couple-three years ago. Proper alignment
made a measured... trying to remember, but I think it was at *least* 20%
difference in throughput.

Alignment's easy: using parted (the user-hostile program), if you do go in
with parted -a optimal /dev/drive, and do
mkpart pri ext4 0.0GB 100% (for non-root drives, for example), it's
aligned correctly.

        mark

On Fri, Feb 27, 2015 at 2:59 PM, Chris Murphy <lists at colorremedies.com>
wrote:
> On Fri, Feb 27, 2015 at 1:53 PM, Robert Arkiletian <robark at
gmail.com>
> wrote:
> > Still have good quality older sata hardware raid cards that require
512
> > bytes/sector. As far as I know HD manufacturers are not making native
512
> > bytes/sector drives any more.
>
> 512n drives still exist, although they tend to be a bit smaller, 2TB or
> less.
>
>
http://www.hgst.com/tech/techlib.nsf/techdocs/FD3F376DC2ECCE68882579D40082C393/$file/US7K4000_ds.pdf
>
>
I too noticed that HGST (now owned by WD) makes native 512n drives. That
pdf states that they come in 2,3,4 TB models. (A6 in the model # represents
512n). But there are almost no reviews on these HGST native 512n drives
online.

> 4Kn drives are appearing now also. I don't expect these drives to be
> bootable except possibly by systems with UEFI firmware. It's also
> possible hardware RAID will reject them unless explicitly supported.
>
>
http://www.hgst.com/tech/techlib.nsf/techdocs/29C9312E3B7D10CE88257D41000D8D16/$file/Ultrastar-7K6000-DS.pdf
>
>
> > Some have better 512e emulation than others. Looking for some advice
on
> > which to avoid and which are recommended. Thanks. PS this is for a
> CentOS6
> > server.
>
> The emulation implementations don't come into play if the alignment is
> correct from the start. The better implementations have significantly
> less pathological behavior if alignment is wrong, but that's
> anecdotal, I don't have any empirical data available. But I'd say
in
> any case you want it properly aligned.
>
According to this pdf [1] alignment is important but from what I understand
512e emulation still has a small RMW performance hit from writes that are
smaller than 4k or if the writes are not a multiple of 4k.

Also it's probably not a good idea to mix 512e with 512n in a raid set.
Although this may be hard to avoid as drives fail in the future.

[1]
http://i.dell.com/sites/doccontent/shared-content/data-sheets/en/Documents/512e_4Kn_Disk_Formats_120413.pdf