## Bug #15653

### crush: low weight devices get too many objects for num_rep > 1

80%

**Description**

### discussion¶

### description, with example¶

CRUSH will correctly choose items with relative weights with the right probabilities for each independent choice. However, when choosing multiple replicas, each choice is **not** indepent, since it

has to be unique. The result is that low-weighted devices get too many items.

Simple example:

maetl:src (master) 03:20 PM $ cat cm.txt # begin crush map # devices device 0 device0 device 1 device1 device 2 device2 device 3 device3 device 4 device4 # types type 0 osd type 1 domain type 2 pool # buckets domain root { id -1 # do not change unnecessarily # weight 5.000 alg straw2 hash 0 # rjenkins1 item device0 weight 10.00 item device1 weight 10.0 item device2 weight 10.0 item device3 weight 10.0 item device4 weight 1.000 } # rules rule data { ruleset 0 type replicated min_size 1 max_size 10 step take root step choose firstn 0 type osd step emit } # end crush map maetl:src (master) 03:20 PM $ ./crushtool -c cm.txt -o cm maetl:src (master) 03:20 PM $ ./crushtool -i cm --test --show-utilization --num-rep 1 --min-x 1 --max-x 1000000 --num-rep 1 rule 0 (data), x = 1..1000000, numrep = 1..1 rule 0 (data) num_rep 1 result size == 1: 1000000/1000000 device 0: stored : 243456 expected : 200000 device 1: stored : 243624 expected : 200000 device 2: stored : 244486 expected : 200000 device 3: stored : 243881 expected : 200000 device 4: stored : 24553 expected : 200000 maetl:src (master) 03:20 PM $ ./crushtool -i cm --test --show-utilization --num-rep 1 --min-x 1 --max-x 1000000 --num-rep 3 rule 0 (data), x = 1..1000000, numrep = 3..3 rule 0 (data) num_rep 3 result size == 3: 1000000/1000000 device 0: stored : 723984 expected : 600000 device 1: stored : 722923 expected : 600000 device 2: stored : 723153 expected : 600000 device 3: stored : 723394 expected : 600000 device 4: stored : 106546 expected : 600000

Note that in the 1x case, we get 1/10th the items on device 4, as expected. For 3x, it grows to 1/7th. For lower weights the amplification is more pronounced.

### detailed explanation¶

The chances of getting a particular device during the first draw is the weight of the device divided by the sum of the weight of all devices. For example let say there are 5 devices in a bucket, with the following weights a = 10, b = 10, c = 10, d = 10, e = 1. The chances of getting e is 1/41 and the chances of getting a is 10/41.

Things get more complicated for the second draw because we have to account for a first draw that does not include a given device: it is the sum of the weight of all devices except the one we're interested in, divided by the weight of all devices. So, if we want to know the chances of e showing up in the second draw, the first draw must not include it and this has a 40/41 chance of happening. Also, during the second draw, the chance of getting e is increased because there is one less device to chose from (the one that was picked during the first draw): 1/31 (i.e. 41 - the weight of the device that was chosen). Because the second draw depends on the first draw, the probability must be multiplied: 40/41 * 1/31.

Since the chance of getting the device e in a first draw or getting the device e in a second draw are independent, the chances of getting the device e in both situations is the sum of their probability: 1/41 (first draw) + (40/41 * 1/31) (second draw).

This is a special case because all devices have the same weight except for e. If we are to calculate the probability of a being selected in the second draw, we have to sum the case where e is selected and the case where b, c or d is selected during the first draw, because they do not have the same weight. If e is selected during the first draw, a will be selected during the second draw with a probability of (1/41 * 10/40). If b, c, or d is selected during the first draw, a will be selected during the second draw with a probability of (30/41 * 10/31).

The chances of getting the device a in the first draw and the second draw is therefore: 10/41+(30/41 * 10/31)+(1/41 * 10/40)

To summarize:

- probability of getting e : 1/41 + (40/41 * 1/31) = .05586
- probability of getting a : 10/41+(30/41 * 10/31)+(1/41 * 10/40) = .48603

We are therefore 8.7 ( 0.48603/0.05586 ) more likely to get e than to get a.

From the point of view of the users, this is counter intuitive because they expect that the weight reflects the probability, which is only true for a single draw. With just one draw a is (10/41)/(1/41) = 10 times more likely to be selected than e. With two draws, a is only 8.7 times more likely to be selected than e, as shown above.

### History

#### #3 Updated by Adam Emerson 9 months ago

**Category**set to*CRUSH***Assignee**set to*Adam Emerson***% Done**changed from*0*to*50*

I just need to put in a special case for LIST, (I don't know if I should just not bother with TREE) test it some, and then everything should be set.

#### #4 Updated by Adam Emerson 9 months ago

**Status**changed from*Verified*to*Testing***% Done**changed from*50*to*80*

#### #5 Updated by Dan van der Ster 4 months ago

Does this issue explain our uneven distribution? We have four racks, with 7, 8, 8, 4 hosts in each, respectively. The rack with 4 hosts (RA13) is getting noticeably more data than the others (as is the 7-host rack, RA01):

ID WEIGHT REWEIGHT SIZE USE AVAIL %USE VAR TYPE NAME -2 3532.61450 - 3530T 1528T 2002T 43.29 1.19 room 0513-R-0050 -72 911.81860 - 911T 399T 511T 43.85 1.20 rack RA01 -4 1048.31836 - 1047T 424T 622T 40.54 1.11 rack RA05 -6 1048.31836 - 1047T 421T 626T 40.21 1.10 rack RA09 -9 524.15918 - 523T 282T 241T 53.94 1.48 rack RA13

I suppose the workaround for this is to decrease the weight of the smaller rack?

#### #6 Updated by Loic Dachary about 2 months ago

**Needs Doc**set to*No*

See https://github.com/ceph/ceph/pull/10218 for a discussion and a tentative fix.

#### #7 Updated by Loic Dachary about 2 months ago

The test Adam wrote to demonstrate the problem, made into a pull request: https://github.com/ceph/ceph/pull/13083

#### #8 Updated by Loic Dachary about 2 months ago

**Description**updated (diff)**Status**changed from*Testing*to*In Progress*

#### #9 Updated by Loic Dachary about 2 months ago

**Description**updated (diff)

#### #10 Updated by Loic Dachary about 2 months ago

**Description**updated (diff)

#### #11 Updated by Loic Dachary about 2 months ago

**Description**updated (diff)