This article describes the pro's and con's of the various connections available for external drives, so you can decide what to choose for your system.
- Last Updated on Wednesday, 29 July 2015 13:50
You need external drives on your system. It can be because there is not sufficient room internally, or because you need to use these external drives on multiple systems. Whatever the reason, be aware of the consequences of the connection used for these drives. The basic question is what do you need them for? For backup, then any connection will do. For off-line storage, then again any connection will do. For near-line storage, then use USB3, eSATA or even Gb ethernet, teamed or even better 10 Gb ethernet, for on-line storage you need much more speed.
Consider this table:
Typical Sustained Transfer Rates depending on the Disk Interface
HDD < 40% Fill Rate
HDD > 70% Fill Rate
|SATA 6G||150 - 170 MB/s||90 - 110 MB/s||300 - 530 MB/s||6 Gb/s|
|SATA 3G||150 - 170 MB/s||90 - 110 MB/s||150 - 250 MB/s||3 Gb/s|
|USB 3*||80 - 100 MB/s||60 - 80 MB/s||300 - 380 MB/s||5 Gb/s|
|FW 800||55 - 60 MB/s||45 - 50 MB/s||800 Mb/s|
|FW 400||35 - 40 MB/s||30 - 35 MB/s||400 Mb/s|
|USB 2*||20 - 25 MB/s||20 - 25 MB/s||480 Mb/s|
* Maximum effective transfer rates. Can be lower with multiple USB devices on the same port.
The thing to remember is that eSATA has the same performance as internal SATA, so it is either 6 Gb/s or 3 Gb/s, depending on the generation of the eSATA bus.
Thunderbolt, which is typically used on Mac's, is a 'new' connection. Well, 'new' is rather doubtful. since it is based on a PCIe-2.0 4x connection with a 10b/8b encoding protocol, that carries a huge overhead (almost 20%) over PCIe-3.0 that uses the 130b/128b scrambling protocol. Thunderbolt is great for laptops. There is no connection faster than that for laptops. It delivers around twice the transfer rate of eSATA, with 10 Gb/s per channel. Effectively it will deliver up to around maximum 950 MB/s transfer rates with 2 or more modern and very fast SSD's in a (r)aid0 configuration. But that is also the downside of Thunderbolt. Adding more SSD's to the (r)aid0 will not improve performance. It is choked. It is almost the same speed an internal (r)aid0 with only 2 SSD's will deliver as a software raid. Nothing more. Thunderbolt 2 delivers around 1350 MB/s maximum transfer rate. Still significantly slower than a dedicated raid controller can achieve.
If you are concerned about on-line storage, the only feasible and affordable solution is to use a PCIe-3.0 8x internal raid controller with SFF-8088 connectors. They offer 24 Gb/s per connector, 2.4 times faster than Thunderbolt for each connection, with a maximum of 7.7 GB/s transfer rate. And the CPU load is much lower than with Thunderbolt. Even with conventional HDD's, that are much slower than SSD's with around 150 MB/s for a single disk, it is pretty easy to achieve transfer rates over SFF-8088 connections that far exceed the Thunderbolt choking limit, as shown in these benchmark results, where a limited array, based on conventional HDD's achieves transfer rates almost 4 times faster than Thunderbolt-1 and 3 times faster than Thunderbolt-2:
So, in terms of performance, the order of choices for external disks intended for editing is the following:
- PCIe with SFF-8088 mini-SAS
One important consideration when choosing a connection for external disks, is related to the cable connection. Only SFF-8088 offers a locking cable and that simply means the risk of data corruption due to a loose cable is very small, but all the other connections are the non-locking type, making them very susceptible to coming loose and then causing data corruption on your external disk(s). If a connections gets loose during a write operation, chances are that data integrity is no longer guaranteed and with non-locking cables that should be a real worry.
Keep in mind that external raids, connected over eSATA or USB3 still suffer from the half-duplex nature of the connection. Those connections are easily choked because all the data need to go over a single connection and their performance suffers greatly. Even Thunderbolt chokes at maximum transfer rates of around 950 MB/s. And remember that all parity raids are ill-advised if not used with a dedicated hard raid controller and a SFF mini-SAS connector.
In a simplified manner it looks like this:
Using SFF-8088 connections is like driving on a four lane highway for each channel in use. Using eSATA or USB3 is like driving on the same highway where three lanes are blocked. You will have huge traffic jams when traffic is more than very, very light. This makes it clear that a single eSATA or USB3 cable to connect the external storage case is easily choked by the connection and can not ever compete with SFF-8088. If the external connection uses eSATA 3G instead of eSATA 6G it is even worse, because the connection bandwidth chokes much earlier. Like adding a low speed-limit on the single available lane.