I read somewhere the Windows 7 had removed the limit on ReadyBoost from 4GB and now allows an unlimited number of GBs. I would like to reconfirm this as a fact.

At present, my Windows 7 RC 7100 allows me a maximum of 4GB on one of my 8GB flash drives (though I've tried to connect two at a time and use 4GB on one and 2GB on another).


  • For win7 > The 4-GB limit of Windows Vista has been removed. technet.microsoft.com/en-us/magazine/ff356869.aspx – kxmp Dec 7 '14 at 10:14
  • I read that link differently: "the ReadyBoost cache cannot be greater than 4 GB on a FAT32 file system or greater than 32 GB on an NTFS file system." The question asked about flash drives, which I have never seen formatted NTFS. – fixer1234 Dec 7 '14 at 16:49

According to this comment for this post at Windows 7 News, someone is using 8 GB for Readyboost on 1 ExpressCard and 4 GB on 1 SHDC card at the same time.

  • Thank you this answers my query correctly. I reformatted my 8GB flash drive using the exFAT format and then ReadyBoost allowed me to configure this drive for 8GB cache. Thanks and did the job! :) – ymasood Sep 7 '09 at 11:24

I also found this on wikipedia :

Windows 7 allows up to eight devices for a maximum of 256 GB of additional memory.


MS lists the 8-device, 256-GB-total ReadyBoost limits here: http://windows.microsoft.com/en-US/windows7/products/features/readyboost


On flash memory device formatted as exFAT or NTFS, a maximum of 32 GB per device, up to eight devices for a total of 256 GB:

Windows 7 allows up to eight devices for a maximum of 256 GB of additional memory, with up to 32 GB on a single storage device.


Note that a device formatted as FAT32 is limited to 4 GB due to the file size limit imposed by FAT32.


Let's things clear for all people:

ReadyBoost is a cache, so any time it reads from cache it gains.

But, have in mind this:

  • If your HDD seek is faster than on your USB drive, no gain is possible with small files
  • If your HDD secuantial read is faster than on your USB drive, no gain is possible with large files

Some USB sticks are very slow, some very good (and also very expensive) HDDs have a seek time of less than 1ms, for that ones, ReadyBoost can not gain anything, since the HDD is faster than any other.

When ReadyBoost can gain? Easy: when HDD is slower than USB memory used.

It has nothing to do with how much RAM is in use... ReadyBoost is used for Read DATA when applications load files, are executed, etc...

But, if you do not have enought Free RAM for all apps loaded, then it can be a 'gain' because Windows tends to use a portion of RAM for such cache... with ReadyBoost enabled such cache goes onto USB, leaving more RAM free.

That is no gain at all, if USB is slow... better use a 4GiB Pagefile on a second Sata HDD... or use a Sata to CompactFlash / SD / etc adapter and use it for pagefile.

Now things clear, ReadyBoost gain is only possible when:

  • ReadyBoost can read files faster form USB memory than form your actual HDD

Having that in mind:

  • Use of multiply USB 2.0 memories (assuming all have same seek time and speeds) is much better than using just only one of similar speed... better use four of 1GiB each than one of 4GiB if all have similar read/write/seek specs.
  • Do a Read/Write test and use the fastest USB memory you have, that would make it as better as possible
  • Without ReadyBoost run 'resmon' to see how much RAM is really 'free' and how much is 'waiting'... the gain can olny be on that size... that would make an idea how much RAM can be freed by using ReadyBoost, so you can load more apps, etc... regardless of speed,... this is only to gain Free RAM, not to improve speed


  • No matter how much RAM you have, ReadyBoost can gain if USB speed is enough
  • ReadyBoost allways gain something... more Free RAM... and if your USB memories are faster than your HDD it will also gain in loading speed
  • Give it as much GiB as you can
  • Use more than just one USB port for ReadyBoost

How many USB ports to use? Easy: Use USB memories of same specs, (like in Raid0), then Sum all Read/Write speeds... if your USB is 2.0, it has a transfer limit of near 25MiB/s (i have never got on Windows more than that with very fasta USB Sata II HDDs on secuential Read/Write, on Linux i can go upto 28MiB/s and with a Root USB port upto 48MiB/s).

In other words:

  • If 'resmon' tells you that your free RAM is Zero (most like can be) and ready RAM is 500MiB, can use ReadyBoost to gain 500MiB free ram
  • If your HDD is too slow versus your USB stick/s (sum USB sticks speed to campare), use ReadyBoost to gain speed when loading files
  • If you use software that constantly reads millons of differente files, do not use ReadyBoost, you will loose all benefict since if can not caché all of them, and the fail versus succed is very poor... for example 3D games framerate lost 100fps downto 40fps or less...
  • More than one USB stick is better than just one, see Raid0, it is similar... if two files need to be readed at same time and are allocated on different USB sticks, can be readed at same time (also see USB chain and ensure thay are located on differente root ports)

Advise: - USB_A_stick read at 10Mib/s - USB_B_stick read at 10Mib/s - If use both on same root port can not read at same time, so on 1 second can read only 10MiB (from US_A_stick or from USB-B_stick)... this is not tottaly true... USB can transfer more than 20MiB/s, so if Reads are with a size less than 10MiB, they can be interlaced, so they can read near 20MiB/s... that needs both USB sticks to have a 'buffer', most have it... old ones, very old ones does not have it. - If use both on different root port can not read at same time, so on 1 second can read 20MiB (10 from USB_A_stick and 10 from USB-B_stick) at same time

So what is better? Test and test it seriously...

Test your HDD seek time, test your HDD secuential and random Read and Write... compare them to the USB sticks, etc you have... see how many USB ports you can use at same time... test them in parrallel so see if can transfer at same time or waits till other in use... etc...

I recomend:

  • Activate ReadyBoost with only one USB stick on one USB port, test loading speeds
  • Activate ReadyBoost with only two USB sticks on two USB port, test loading speeds on different USB port
  • Test with three USB sticks... four, five... etc... as much as you can
  • See how much free RAM you gain
  • Evaluate the resoults and act acording to them Note: Between test, please power down the PC, so ReadyBoost does not have anything cached, so that way results are not getting false.

It is not easy, but it is worth a try.

If your HDD is a Sata III, able to read at 300MiB/s or more, with a seek time of 2ms, no gain will be if your USB is 2.0, so in that case only use it if no free RAM, run remon to see real Free RAM versus 'waiting' ram.

ProcessExplorer only gives 'Free' RAM, it does not show 'waiting' ram.


I don't know about "somewhere" but the Wikipedia ReadyBoost article supports that:

Windows Vista x86 is limited to using 3.5GB (Vista x64 can support up to 16GB); this restriction has been removed in Windows 7.


The initial release of ReadyBoost for Windows Vista supports one device. Windows 7 supports multiple flash drives for ReadyBoost.

However, I can't find anything on the Microsoft site, which isn't encouraging.

  • Multiple flash drives sounds nice, didn't know of that yet (if it's true), thank you! :-) – Tamara Wijsman Sep 7 '09 at 9:27
  • I wonder how much overhead the encryption uses on Readyboost? I've often wondered if Readyboost could be made faster if there was the option to disable the encryption. – Matthew Lock Feb 17 '14 at 2:00
  • 1
    @MatthewLock: No idea, I'm afraid :( – Jon Skeet Feb 17 '14 at 6:45
  • Did some further reading and apparently Microsoft allows manufacturers of internal non-removable readyboost devices to disabled encryption, so I assume from that there's some performance benefit. – Matthew Lock Feb 17 '14 at 9:59

Official information from Microsoft can be found on the Windows 7 Product Website with additional information available on TechNet. I'm sure some searching might reveal even more information from Microsoft, but for a start this should be sufficient.


To know: 'waiting' ram is RAM that is not Free but can be Free inmediatly.

Windows will say Free RAM is zero (at least on my 2BiG), but has more than 333MiB of wainting RAM, that also with ReadyBoost working.

Final conclusion on 32Bit:

  • better use ReadyBoost with as fast USB sticks as possible on as much USB ports as able to improve speeds
  • and a 4GiB pagefile allocated on a dedicated HDD as fast as possible, or at least on a dedicated FAT32 partition, never as a file in system partition.

Final conclusion on 64Bit (with a lot of GiB of RAM):

  • Use a RamDrive for PageFile
  • If your HDD is fast do not use ReadyBoost

That is what i can say based on my own 'tets'.


ONLY if you have LESS THAN 2 GB of RAM on your computer, enable ReadyBoost for your USB flash drive to cut down on the number of hard drive accesses Windows makes.

A 512 MB to 3 GB ReadyBoost.sfcache file should be adequate.

Tests have shown that there is no benefit to using ReadyBoost if your computer has > 2 GB of RAM and there is no need to use more than one USB drive. If you have a spare 4 GB or less USB flash drive you could use the whole drive and plug it into the back USB port for aesthetic reasons.

  • 1
    not true, the benfit of ReadyBoost is when you don't have enough ram to hold all the dlls the apps your are running make use of, so it very match depends on your workload. However if you have under 2GB of ram and can't upgrade then ram, then get ReadyBoost TODAY! – Ian Ringrose Jul 18 '10 at 8:25

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