Hi,
This is an interesting one; I sell hard drives so I have to deal with
this sort of thing every now and again. I'll try and give some
background to the problem below:
Traditionally, hard drives have had a sector size of 512 bytes. This
worked well for a long time, but it's not really optimum for larger
sized drives. This is mainly due to ECC data - there are other pieces
of information stored in sectors, but ECC is probably the most
well-known so I'll stick to talking about ECC in this email. Each
sector has a certain amount of bytes reserved for ECC, so on larger
drives, the amount of capacity that is reserved for ECC really starts
to add up.
To solve this, hard drive manufacturers shifted to a 4K sector size
instead. If the same ECC algorithms are used, to put it simply, much
less space is taken up by ECC due to there being fewer sectors on the
drive - 4096 / 512 = 8, so in theory 8 times less.
To try and ease the transition, manufacturers introduced what are
known as advanced format or AF drives where the physical sector size
is 4K but for compatibility reasons, the drive will usually (more on
this later) show a sector size of 512B to the OS. Unfortunately, just
to make things a bit more confusing, there are a *very* small number
of drives available now that don't perform the 512B emulation - E.G.
they expose 4K sectors to the OS by default, but these drives are also
labelled as advanced format drives.
The problem (in fairly simplistic terms) is that most applications
only deal with logical sectors, so sometimes write operations will
fail. Let’s suppose that some form of application exists that stores
data in 512B "chunks" due to it being written before 4K sectors were
introduced. On a drive that has physical 4K but emulates 512B, 8
"chunks" of data will be stored per physical sector. Unfortunately, if
some form of corruption were to happen, potentially 8 "chunks" of data
are now lost instead of one.
I'm not sure how the backup program you're using works, but imho the
problem probably relates to it assuming 512B physical sectors but due
to the API method it's using to write files*, on drives that have
physical 4K but emulate 512B it's getting confused. *There are
multiple API methods to handle reads & writes; some return the logical
sector size but some return the physical size, so compatibility will
depend on what method is used.
The best root to go would be for you to make your own external - E.G.
buy a drive & a USB enclosure so that you can be sure that the drive
you're using is 512B physical. Any Western Digital drive with an "a"
as the second character of the block of 4 letters after the numbers
will be 512B physical - for example, some 2TB drives that should work
are WD20EACS, WD20EADS, WD20EARS, WD20EACX, WD20EADX and WD20EARX.
Anything with a z instead of an a is physical 4K with 512B emulation -
E.G. WD20EZRX won't work.
Hope this helps.
Cheers,
Ben.
On 6/26/14, Andrew Hodgson
Aha,
I used 1TB drives on SBS as it compressed everything so I could get quite a few incrementals on it.
It looks like Western Digital have a solution to this problem:
http://wdc.custhelp.com/app/answers/detail/a_id/6618
Not sure whether you can do something similar in Seagate drives.
Andrew.
-----Original Message----- From: Blind-sysadmins [mailto:blind-sysadmins-bounces@lists.hodgsonfamily.org] On Behalf Of George Bell Sent: 26 June 2014 11:06 To: Blind sysadmins list Subject: Re: [Blind-sysadmins] USB External Server Backup Drives
Total on all drives is around 800 GB, so I'd figured that 2 TB would allow for that plus incrementals.
Up until now, I'd been using a 300 GB Maxstor USB drive, and just backing up the System, not the data.
This is a known issue, and below I've pasted just one easy to understand explanation.
Allegedly some of these larger drives do support 512k sector size, but it's all but impossible to find out which. Hence my asking if anyone was currently using one and what make and mode.
George.
"The reason is that most of these large drives have a 4K sector size, rather than the older 512 byte sector size. You can get a patch for Windows 7 and Windows Server 2008 R2 that fixes most problems, but it does not fix the backup issue.
The further reason is that Windows Server Backup uses VHD virtual drives as its file format. The VHD format presumes the use of 512byte sectors, and the drivers that read and write data are optimised for this. You cannot create or mount VHDs on a disk with 4K sectors."
-----Original Message----- From: Blind-sysadmins [mailto:blind-sysadmins-bounces@lists.hodgsonfamily.org] On Behalf Of Andrew Hodgson Sent: 26 June 2014 10:04 To: Blind sysadmins list Subject: Re: [Blind-sysadmins] USB External Server Backup Drives
Hi,
How much data are you backing up? I used standard USB drives and they worked fine.
Andrew.
-----Original Message----- From: Blind-sysadmins [mailto:blind-sysadmins-bounces@lists.hodgsonfamily.org] On Behalf Of George Bell Sent: 26 June 2014 09:56 To: Blind sysadmins list Subject: [Blind-sysadmins] USB External Server Backup Drives
Does anyone know of, and perhaps even use, a make and model of UEB external drive which will allow backup of SBS 2011/Server 2008 R2. Ideally I need 2.
There appears to be a very limited range of such drives, and certainly the likes of the Seagate Backup Plus (2 TB) do not work.
It needs to be something I can remove one from the premises, without getting a hernia in the process.
George. _______________________________________________ Blind-sysadmins mailing list Blind-sysadmins@lists.hodgsonfamily.org http://lists.hodgsonfamily.org/listinfo/blind-sysadmins
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